Tock Embedded Operating System (tockos.org)

Interesting, apparently an 8 years old project - never heard of it (though this doesn't mean anything beside that fact that I might be living under a rock).

Why should one choose this over Zephyr for example? I don't see a lot concrete info about its benefits after a quick glance

I have never heard about it either, seems they have put quite some effort in it, but i can't find much apps using it.