Whatever the case, in most cases of abhumans, the warp had nothing (direct) to do with it. Ogryns, Squats, and Ratlings are all evolutionary offshoots that developed on specific planets and specific natural circumstances. They're not mutations by "modern" imperial standards (although many might surely consider them as such, simply because they aren't of the norm and the vast majority of the Imperium's populace is ignorant).I don't know if I'd call it "basic" evolution. That sort of thing doesn't happen over the course of ~10.000 years. More like they were radically altered by exposure to warp energies and/or radiation (though I don't believe radiation-based mutations usually carry over to the next generation, so warp seems a lot more likely - but then again, comic book logic) and the abominations that resulted eventually sexed themselves into a fairly homogenous shape.
From our point of view in the real world, the Ogryns came about through mutation and adaptation, and, basically, basic evolution. By our definitions, they aren't really mutants, and they aren't mutants in 40k either. They're an evolutionary branch of humanity, or, in 40k terms, abhuman.
I suppose that could be called evolution (my next genetor will definitely write a large number of dissertations on "unnatural selection") but hardly basic.
Basic evolution absolutely happen over the course of 10,000 years (or less), though, although the process might be hard-pressed to result in such large differences in such a short timespan. Evolution is really just a process of favoured reproduction and epigenetic influences. Modern life has stifled this to an extent in humans, but if these populations were cut off from the rest of humanity, odds are only a few survived and lived to evolve and repopulate their planets.
And it's likely closer to 40 000 years, anyway. Still short, from an evolutionary perspective, but potentially more than enough. Abhumans haven't evolved over the last 10 000 years, it's likely been going on since humans got there, likely in the beginning of the 2nd millennium or something.
Anyway, I'm rambling. My point was that the warp didn't create abhumans, except arguably psykers, if you consider psykers abhuman. From an objective in-universe point of view, tl;dr: evolution did it.