Every single other virus ever has had a zoonotic origin. SARS-CoV-2 has a 96.3% homologue.
The default, as with literally every other of thousands of viruses, is that like all the others it is the product of zoonosis.
For most viruses too, the exact intermediate animal hosts aren't precisely known.
It's really akin to find a new species of bird and trying asking for evidence that it specifically came from evolution and not genetic engineering. Sure, it's possible that it didn't, but every single other species of bird came from it and it's fully expected to happen.
And by the way, the exact intermediate host for the original SARS was also never determined conclusively until fifteen years after the first outbreak.