It seems plausible, if not exactly likely, that Tommy might want to wait for a full 60-120 seconds; in context, he's doing something sneaky, and because he presumably doesn't want to get caught, he would be justified in being cautious.
That aside, your assumption that "minute" isn't a definite amount of time seems fair. And the use of "minute" would be colloquial, not amateurish or mistaken.
This ngram shows that the use of "moment" was declining while Christie was writing, and has declined markedly since her death. If anything, her substitution of "minute" for "moment" seems to anticipate a broader trend in the language.
According to the OED, as early as 1393, a "minute" was
The space of a minute (also minute while); a short space of time, an instant, a moment.
If usage of minute for moment was acceptable then, I think it's safe to assume it was fairly standard in Christie's time.
The meaning of garble as to distort began as a usage of its meaning "to sift" as to pick out parts of a speech or writing, as we would now say to "quote out of context" or to "cherry-pick".
By 1930, the word had begun to be used generally as "to confuse", but the meaning of "to cherry-pick quotes" was still common.
Since about 1950, the meaning has rapidly changed, being used as a term for radio interference, and for general confusion of meaning.
So we see a progression from
- Select the good parts of something
- Select parts (good or bad) to serve a purpose (by 1800)
- misrepresent an idea, message or author by quoting out of context
- Misrepresent an idea, message or author deliberately or accidentally (by 1930)
- generally confuse a message of any kind (by 1930)
In the 1930s, both meanings were in use. After 1970 at the latest the last meaning has become almost exclusive. It seems to me that the most likely reason is the adoption of "garble" as a jargon term to mean radio interference, combined with the exposure of large portions of the population to radio communication during the second world war.
Discussion
In the early 1800's the meaning of garble as "to take out of context", generally deliberately, is well supported:
Johnson's Dictionary (1805) has:
To GARBLE v. a. [garbellare, Italian.] To sift; to part; to separate the good from the bad.
But you, who fathers and traditions take, And garble some, and some you quite forsake (Dryden)
Had our author set down this command, without garbling, as God gave it, and joined mother to father, it had made directly against him (Locke)
Mr Pole's Justification of the arrest of the Catholic Delegates (1811):
I shall however, deal by his work with candor; I shall not garble his lines nor his sentiments; I shall not pluck from his pages the passage where the author may have slept, nor shall I take advantage of the page which he has written in the cause of liberality, by only quoting that which he has written in the cause of intolerance. I shall deal fairly by the author...
Cobbett's Political Register, Volume 19 (1811)
It never has been my custom to garble, or to suppress. I always like that my opponents should be heard as well as myself.
Remarks by E Burke on mr Stanser's Exammination (1805)...
... in imitation of other pamphleteers collects a summary of what he calls Catholic doctrine, not from Catechisms, Manuels, Professions of faith, or any authentic source ; but from the misrepresentation of party writers, who finding it impossible to refute any article of Catholic doctrine fairly stated, garble some quotations from obscure writers of no authority
An Essay on the Law of Patents for New Inventions (1810)
I agree that in so doing we must not garble any sentence referred to, so as to give it a different meaning from that which it naturally imports, when taken altogether;
Around this time I don'[t find any examples of "garble" as to "accidentally misunderstand or misquote".
Between 1850 and 1859 we find the same:
Notes and Queries, volume 9(1854)
As usually applied in England, to garble is to pick out, sift out what may serve a particular purpose, and thus destroy or mutilate the fair character of the whole.
Webster's Dictionary (1854):
- In present usage, to pick out or separate such parts from a whole as may serve a purpose
The Parliamentary debates (1854)
with expressions, it is not to garble despatches, it is not by quotations here and misquotations there to make n colourable case of mismanagement against the Ministry, at a moment of national exigency like the present, that I am led to quotation
From 1900 to 1909:
The Fortnightly Review - Volume 80:
Did he, in other words, in describing Carlyle's behaviour as a husband, garble his evidence, and suppress or distort facts, in order to represent the character of his dead friend as worse than it really was
By 1950 the meaning had begun to change.
Still in the old sense:
Encyclopaedia Britannica (1952)
GARBLE, originally a mediaeval commercial term in the Mediterranean ports, meaning to sort out, or to sift merchandise, such as spices, etc., in order to separate what was good from the refuse ; hence to select the best of anything.
And in the new sense:
Stories for Tomorrow: An anthology of modern science fictio (1954)
He turned on the recorder and winced at the garble of sound that blared forth
By 1975 most examples seem to be to do with radio interference.
Best Answer
Salsburg appears to be wrong.
The OED gives meaning 2: "That has or conveys a particular meaning; that signifies or indicates something." from 1573; and meaning 4a: " Sufficiently great or important to be worthy of attention; noteworthy; consequential, influential." from 1642.
He might of course be right that meaning 4a was less common until the 20th century: the OED does not tell us that. But it is not true that it didn't exist until then.
The semantic shift seems very natural and unsurprising to me.
The OED dates the statistical sense from 1885.