It all ties back to the complaints (at the time) about how D20 was slowing innovation because so many people were doing D20 stuff.
I was never someone who made such complaints, and in retrospect I don't think I was mistaken.
That period saw HeroWars/Quest, Burning Wheel, the emergence of Vincent Baker as a prominent designer, etc. These are all innovative RPGs, some of the most innovative since Traveller, Runequest and Champions 35 or so years ago.
"gamer" is a pretty vague term. WotC made it very clear at the time that they felt like the "gamer" community was vastly bigger than the TTRPG-gamer community and they thought they could change that.
In the general sense, of course 4E was intensely "gamer-focused". But the vast majority of non-TTRPG gamers remained non-TTRPG gamers.
In the TTRPG sense 4E was highly gamer focused if you were in the niche it fit, notsomuch if you were not. Which of course means the term ends up not having a useful definition for this use.
"Gamer" is a vague term. The main thing I had in mind is that 4e is an RPG based on game play - if you don't care about weighing mechanical options, crunching mechanical outputs, etc then it is not for you. In that respect it is closer to (say) Burning Wheel than (say) Moldvay Basic.
To pick two systems somewhat in the middle of that spectrum, let's take RQ and 3E.
Both RQ and 3E can be quite crunchy if you want them to be. But in those systems you can also look down a character sheet, see a bunch of bonuses/skill numbers (+X to this, Z% that, etc) and thereby form a mental 'picture' of the character. The skill/ability names, plus associated numbers, can fairly easily translate into loose descriptors for the character. Thus, for those who want to play with less mechanical crunch and more free narration, RQ and 3E are (I conjecture) fairly viable systems. Rather than (say) using some complex mechanism for Bluff checks, you look down the sheet, see that the PC in question has +20 to Bluff, and the GM determines that the NPC is fooled. Or you see that the PC has 18 STR, and so yes, s/he can kick the door down if s/he wants to.
4e isn't like that. You can't get a clear picture of who a character is, or what s/he can do (especially but not only combat-wise), without mediating the stats through the game mechanics (either in play, or in imagination). 3E can trend in this direction the more that complex feats and unfamiliar spells are part of the game, but 4e defaults to this.
Someone who is not a gamer - as in, not interested in game mechanics (playing with them, imagining their operation) - is, I think, likely to find 4e unsatisfactory. One limit case of this is people who play 4e and declare only basic attacks for their PCs. Such a person is having no experience - cognitive, emotional - of what his/her PC is actually about, who his/her PC actually is. A very rough analogy would be someone who sits down to "play" chess but only moves his/her pawns and gets confused about what is going on when the opponent castles, or takes en passant.