It's worth noting that other early games did not use XP.
Timeline
1975 Tunnels and Trolls uses either AP (Adventure Points) or EP, depending upon edition.
Runequest (1976) and Traveller (1977) didn't use experience points at all.
1974 & 1976 † Original Edition D&D doesn't use XP nor EP. Electrum are mentioned, but non-standard, and Experience is spelled out.
1978 Starships and Spacemen, by FGU, abbreviates Experience Points to Exp. Pts.
1978 Holmes Basic D&D (Blue cover) spells out Experience, but does use EP for Electrum Pieces.
1978 AD&D Player's Handbook uses E.P. on page 20 (in the Cleric description) but nowhere else, and spells out experience everywhere else it's used.
1979 AD&D Dungeon Master's Guide uses X.P. and XP (many pages, defined on p. 230), and defines EP and E.P. as Electrum Piece (p. 228)
1981 Mechanoid Invasion (Palladium) uses Experience, spelled out.
As noted, 1981 Moldvay Basic D&D uses XP and EP.
Discussion
Therefore, Moldvay is not the origination of the use.
Moldvay also was not in print as long (it was replaced in 1983) where AD&D was in print with literally only cosmetic changes until 1988‡ or so.
Moldvay was quite popular, while it lasted, but had less lasting impact.
Therefore, it is most likely that AD&D is the popular source, not Moldvay's Basic D&D. Further, looking at many early videogames using XP, like Final Fantasy, the classes are closer to AD&D; the inclusion of Rangers, Assassins, or Druids, and separation of Race and Class are classic features of AD&D that were absent in Moldvay.
Footnotes
† 1976 revised some wordings and renamed several monsters due to a lawsuit from Tolkien Enterprises.
‡ AD&D 2 was announced, and AD&D 1 was no longer in print. 1E would be found in stores for several years on, as stocks in distribution continued to sell.
Really, experience points are just a game mechanic, used to incentivize and/or reward certain behaviors
As noted in the passage you quoted, they are meant as rough indicators of the experiences that help a character learn, grow, and improve herself...but any close mapping to how real people learn and grow is tenuous at best.
Would apprenticing with a high level wizard help you master new spells as quickly as shooting orcs with magic missiles day after day? Perhaps, but it wouldn't make for an interesting game incentive.
Some GMs assign XP only (or primarily) for defeating monsters. Others use them to reward clever solutions to problems and/or great roleplaying. In either case, looking too closely reveals that XP are really just a means to incentivize and/or reward certain behaviors.
Pathfinder without XP
As a side note, at least half the Pathfinder (and other 3.x) games I play nowadays don't actually use them; the party just levels up when it fits the story.
In general, I'm a fan of doing away with XP in Pathfinder, but there are downsides to doing so. The main advantages I see of using XP versus simply leveling by GM fiat are:
- XP provides visability which some players will appreciate (e.g. I've accumulated 4226 of the 5000 XP I need to get to the next level), and
- Assigning XP values by the book may avoid arguments with players who think they should certainly have levelled up by now.
Best Answer
The ultimate and most ancient point system for "quantizing success through a numerical method" is, of course, money. Or perhaps predating even that, number of cattle, sheep, size of land controlled, etc. And war and trade were very early human activities to optimize that quantized success.
History aside, LordVreeg's answer above looks the most promising to me so far in terms of modern gaming.
Update
Since my original post, I have found a online-viewable citation that clearly credits David Arneson (co-creator of D&D with Gary Gygax) as creator of the "experience (point) system". It comes from a March 2008 Wired Magazine article, "Dungeon Master: The Life and Legacy of Gary Gygax" by David Kushner, and was written shortly after Gygax died (2008). It describes the creation/development/motivation of the experience point system this way:
It does seem to be clear from this point that experience points were not in Chainmail and were Arneson's invention and part of his contribution to D&D. It does not necessarily rule in or out whether Areneson pulled the idea from somewhere else however.
Your question and other comments do leave me curious to know about ancient competitions (e.g. gladiators, tournaments such as jousting tournaments)-- whether they had any cumulative or longer-term rankings (based on win/loss ratios or #wins) that would be quantitative and cumulative in nature and thus similar to XP beyond what you'd just find in "sports".
I think it's a great topic worthy of intellectual and historical research. What you call "currency of achievement" is what I think of as a "Cumulative point system". As I've reflected upon my own behavior and others' over time, and beyond just the sphere of "games", I've found that cumulative point systems-- whether found in games, online forum reputation systems, or real-world systems like money -- tend to have three great virtues that make them addictive-behavior generating:
Good luck with your research!