It's absolutely a fiction, but at the same time, this at least attempts to be a standard.
The WHATWG version seems more like a reflection of "oh by the way that's the rules our browsers are following this month. Your's truly, the browser vendors."
So far we've done our best to keep a really readable, tidy Git commit log, which should help with understanding changes: https://github.com/whatwg/html/commits/master
If you omit the "Editorial:" or "Meta:" commits, I think it's actually at a similar level of detail as the W3C fork's changes log. (Not completely; scrolling through I do see a number of commits that wouldn't be relevant.) But the W3C fork has only managed to copy-and-paste a small subset of our changes, so indeed, the changes log for the last year of work at the WHATWG will be somewhat daunting compared to the small subset they managed to copy over.
There may be room for someone to compile a higher-level "this week/month/year in the HTML Standard" or similar; before I started working in the WHATWG, that actually used to exist: https://blog.whatwg.org/category/weekly-review (also in very amusing YouTube form: https://www.youtube.com/watch?v=1Bg5BPnmj68). So far we haven't had the bandwidth to restart that, but if you or someone else wants to contribute that sort of thing to the blog or elsewhere, I'd love to help you get started.
I don't think the commit history works. It doesn't give you any indication about which changes are relevant or irrelevant and it doesn't tell anything about the larger efforts taking place.
Actually, I don't think it would even make sense to create an equivalent of the W3C diff, because there are no versions or other structures to organize the changes around - there is just a constant stream of changes. (Which is kind of the point of the living standard concept after all)
Personally I'd tend toward weekly or monthly, although I admit that yearly is more likely to generate HackerNews posts ;)