1) The specs are now much more publicly accessible than they used to be, and frankly the spec does a fairly reasonable job describing a tricky format relatively compactly. It requires a fair bit of knowledge of Photoshop to read and understand, but it’s mostly fairly explicit. Much better than many other proprietary document formats.

2) For someone with relatively extensive knowledge of photoshop, the format is fairly comprehensible, albeit complicated. The biggest part of the problem here is, as you say, that Photoshop just has a ton of features to support, so that becoming enough of a Photoshop expert to understand it all is a difficult undertaking by itself.

3) The code this comment is taken from only interacts with a small fraction of PSD features, and is frankly pretty awful code: hacky, ad-hoc, not modular at all, etc.

All that said, if someone was to redesign PSD format today, I’m sure it would be organized quite a bit differently, and would have much better re-use of a smaller number of features. (The same goes for Photoshop itself.)