You've smoothly shot right past solid ground (evolution doesn't design things) to a fairly wild claim (rock-throwing apes didn't experience evolutionary pressure to get better at throwing rocks).
Orangutans aren't humans, so what are you saying, apes which don't throw rocks to kill their prey don't... throw rocks to kill their prey?
The poster didn't say "hands", they said "your hands".
Your human hands, which co-evolved with throwing things, we have abundant paleontological evidence of this and you may read as many papers on the subject as you would care to.
Like complex brains are evolutionarily fit because they can solve lots of disparate problems, hands are evolutionarily fit because they can do lots of disparate things.
Of course, we do also then design violins for our specific dexterity.
