Why Can't Xylarthen be a Fighter I?

On  p. 10 of Men & Magic, the first of the original “three brown books”, published in 1974, Gary Gygax introduces the “supposed player” Xylarthen to show how one creates a character.  Xylarthen’s ability scores are these:

Strength              6

Intelligence       11

Wisdom              13

Constitution      12

Dexterity             9

Charisma            8

What class does Xylarthen choose?

This supposed player would have progressed faster as a Cleric, but because of a personal preference for magic opted for that class. With a strength of only 6 there was no real chance for him to become a fighter.

Now, this is a very different sort of character from the ability score superhero that would later become so common.  Gygax himself would say a few years later that unless a character had a 15+ score in at least two abilities he would be “hopeless”.

Under the original rules set—with its scaled down hit points, lower damage (for characters and monsters) and lack of many explicit bonuses or handicaps for those with exceptionally high or low abilities--such an “average” character would be perfectly respectable and viable.

But why then not go all the way with that?  Why can’t Xylarthen be a fighter?

One answer is that with a Strength score of 6 it looks like he would suffer a 20% penalty on earned experience.  That’s the only penalty, by the way, in the three original books of OD&D for having a low Strength—unless one counts being possibly worse at opening doors, worse at “opening traps” (whatever that means, exactly), and inferior at facing off in a test of mental and physical power against lycanthropes, magic swords and the like.

But in fact, he wouldn't suffer a 20% penalty to experience.  Under the ability swapping rules and "for the purpose of experience only", he could use 2 points of Intelligence and 3 points of Wisdom to  adjust his prime requisite score upwards to 8 and thus only suffer a 10% penalty.  Is that so bad?  I don’t think so, especially if one had, say, a “personal preference for fighting.”

So what’s going on here?  Does Gygax in 1974 have some set of rules that he’s sitting on—Greyhawk, perhaps—that makes it more explicit why a fighter wouldn’t want a low score in Strength?  Or is he counting on referees to come up with their own rules and/or rulings to make abilities more meaningful? Or is it just that he thinks a 20% (or even 10%) loss in earned experience would be intolerable?  Or is the whole thing just some sort of mistake or oversight?