I'm not sure that I fully agree that, even by that criteria, the goal was absolutely met. It's not that difficult to make a character who is difficult to hit -even for creatures at your level.
Well, yeah, with the right items and build choices, you can make your character impossible for some monsters to hit, but that doesn’t just happen automatically as a result of gaining enough levels, which is I
think what they meant about bonuses “not being an assumed part of character advancement.” Also, while you can push your bonuses high enough that you can’t
miss, you should never encounter a monster you can’t
hit. As far as I’m aware, the highest AC on a WotC-published monster is 23, which any 1st level character can hit as long as they’re proficient with their weapon and don’t have a penalty to the relevant ability score.
It may be the case that has been addressed by newer books. Admittedly, I wouldn't know. I'm a few years behind on keeping up with 5e. However, I would guess that floating ability scores and more species with inherent defenses makes it easier to build a character that monsters struggle to hit.
There’s a difference though between building a character that monsters struggle to hit, and the situation you had in 3e and 4e, where every character who lived long enough would get to the point where some monsters literally could not hit them ever. With bounded accuracy you can still achieve the fantasy of being nearly untouchable, but doing so requires a commitment of resources, and even if you do commit those resources, you still have to respect the fact that enough kobolds (or whatever)
could still take you down.
By no means is it anywhere near some issues that 3E had (or even 4E had due to early monster math being wonky,) but I think there's still a gap between what was stated as a goal and what was produced.
Perhaps it does just boil down to miscommunication of design goals. I can't say. But the OP isn't alone it having a bit of disconnect.
I think it’s pretty indisputable that WotC did a poor job of communicating what they were going for with the whole “bounded accuracy” thing. I do think they succeeded in what they were actually trying to do, but even at the time there was a lot of confusion about what that was. And it’s really only meaningful in contrast to 3e and 4e, so to newer players it’s got to be even more unclear what this “bounded accuracy” waffle is even about. PF2 is probably the more relevant point of comparison for players who started with 5e.