He says verbatim his goal is to be a stepping stone for serious engine programmers (not necessarily "general purpose", but definitely AAA-level), even though obviously we are not going to be making a AAA game for obvious reasons :)
So fair enough, but between that and his claim that he's showing people how to create a "professional quality" game I think the difference between a AAA game and a "AAA-level" game engine has no distinction, it's basically the same thing.
Is what he has done so far professional grade, capable of "AAA-level" games? I don't think so, but I concede that probably depends on what your parameters for "AAA" and "professional quality" are. It might be fine for indie games but it seems to me that Casey was selling an audience on revealing something deeper.
Part of the purpose of the series of Handmade Hero is to build a game from the ground up with no dependencies. So I don't think he's bringing in stdlib
So he could use that, but goal is to be someone who doesn't necessarily need to do that.