No, therefore should not be reserved for conclusions beyond a reasonable doubt. It is merely a transition similar to thus or as such:
Therefore, I ordered pepperoni.
Thus, I ordered pepperoni.
As such, I ordered pepperoni.
The extended conversation could have been:
I like meat on my pizza. Therefore, I ordered pepperoni.
You can test for an appropriate use of therefore by flipping the sentence order and using because:
I ordered pepperoni because I like meat on my pizza.
The contention that the therefore segment should be restricted to perfect logical use is forgetting that the word really only serves to draw a causal link between statements.
I like red. Therefore, I painted my wall red.
There is no logic here. I am just explaining why I painted the wall red.
Now, if someone is using therefore as a logical link and the logic sucks, you can say that the argument is bad. But the use of therefore isn't the problem.
This $10 item is 50% off; therefore I am saving $6.
This is wrong, but the use of therefore isn't incorrect because it is simply communicating the thought. The communication is accurate; that makes therefore the appropriate word. Replacing therefore with a different word changes the meaning of the sentence (and could correct the logic) but the intent of the speaker no longer matches the communication.
Edit: Since there seems to be some confusion about the actual definition of the word, here is what my dictionary says:
for that reason; consequently : he was injured and therefore unable to play.
Reason, in this context does not mean "logic". It just means "why".
Why couldn't he play? / He was injured.
He was injured and therefore unable to play.
He was unable to play because he was injured.
It is worth noting that there is a strict logical use for the term therefore that explicitly means something akin to "logically derived from the previous statements" but that would be applicable to formations of the following:
All men are mortal
Aristotle is a man
Therefore, Aristotle is mortal
This is commonly represented by three dots in a triangle (∴). But even in this case, the use of therefore is a signal of a specific meaning. If the conclusion is false, it was not an incorrect use of therefore but simply faulty logic. Removing or changing the word doesn't make the problem go away.
You are correct; Karan of the superuser question you referenced made a grammatical error. It should have been as you phrased it:
Not everything in DOS is plain-text
That being said, your understanding in the two examples you posted is slightly off. It should be
Every human is not a man. There is no human being who is a man.
Not every human is a man. There are human beings who are not men.
Such logic mistakes (especially with double negatives) are very common even among native English speakers. The fact that you are able to discern the intended meaning of Karan's comment, even with its incorrect phrasing, and your excellent grammar in this post tells me that you do not at all have a fundamental misunderstanding; on the contrary, your english seems quite good.
Best Answer
Just saying “between each letter” raises the question of between each letter and what other thing? Between always involves more than one thing.
What you mean here is clear enough to most readers, but perhaps it would be more felicitous to write “between each pair of letters” — or more simply, just “between letters”.
Either of those work to provide between with more than one argument. However, don’t worry about the arguments provided being exactly two; they don’t necessarily need to be. That’s a myth. It is a fallacy to assume that between has only two entities. It simply is not true.
Quoth the OED:
Only the case-of-one doesn’t make any sense:
All the other cases — including all your nieces and nephews and cousins and aunts and uncles — are all of them just fine.