Sure, technical documents are long but that still doesn’t support the original claim that they are “unnecessarily complex, bloated, convoluted” and it’s actually evidence against the assertion that they’re “difficult to implement without specific knowledge of its features”: most of why those are long documents is that they carefully detail how necessarily complex systems interact in sufficient detail to implement them whereas the Office XML specs at least historically had things like flags telling to behave like, say, Word95 without fully specifying the behaviour in question.
The original claim was clearly actually just an opinion; I don't think there's merit to treating it as a series of logical statements, or at intricate depth in general.
Evidence for this is in the very words used: unnecessary, complex, bloated, convoluted. These are very human terms that are thus subject to personal interpretation and opinions.
It shouldn't be surprising then that their "claim" thus fails scrutiny. All they actually meant to say is that HTML and CSS are both verbose standards with a lot of particularities - still something subjective, but I think page / word / character counts are pretty agreeable attributes to estimate this with in an objective way. Hence why I brought those up exactly.
Of course it’s an opinion: the point is that it’s neither persuasive nor internally inconsistent. They haven’t given any reason to believe they have enough domain knowledge to compare the two authoritatively. It’s also inconsistent to criticize OOXML for being difficult to implement without extra knowledge and then to criticize a truly open spec for being detailed enough to implement without extra – the entire HTML5 process was intended to reduce the number of cases where people were relying on things which required implementers to know how a specific engine like IE worked.
Sure the spec might be enormous but you don't need to touch it at all to be productive quickly. In no HTML or CSS tutorial i'v ever seen was a reference to the spec nor did i need to go there to solve something. And that in itself is another proof how nicely it is designed actually. Because on the other hand there are other document types or schemas where you absolutely have to go to the spec because it's is so cryptic and badly designed and not self-explaining that there is nothing else you can do.
Wordsmithing your way around this doesn't make them any easier.