I wouldn’t say it’s about forcing yourself to bend the elbows at 90 degrees rather than being in a position in which your elbows are naturally relaxed at approximately 90 degrees, give or take. It’s definitely worse if the angle is sharper, but I haven’t been comfortable with my arms straightened much further beyond that either. I suspect gravity’s impact on blood flow, perhaps it’s individual.

Split keyboards are definitely a rabbit hole one might want to explore, but it’s just logistically difficult to find a good unit that Just Works(tm), is configured or easily configurable as one would expect, doesn’t incur extra latency, is compact enough if you want to work from somewhere else, has key caps you want, is in stock, etc. Meanwhile, external mechanical wireless keyboards with good Cherry switches are available from the usual retailers these days and they take you 90% of the way if you’re currently working with a laptop as is (where your screen cannot be anywhere other than at the level of your hands and vice-versa). Thus, to me the returns of split keyboards are diminishing—to anyone who has wrist or other working posture-related issues and works on a laptop without a dedicated keyboard, I’d say fix that ASAP before starting to geek out on splits.

PS. To be honest, based on my experience, on some of those photos people’s positions look very uncomfortable for every other reason besides the split.