It’s like if there is a massive nuclear waste spill right next to a nuclear power plant. Doesn’t take Kojak to figure out who the prime suspect is...
Many diseases can be passed directly from bats to humans. You don't need an intermediate host to catch rabies, Ebola, Marburg, histoplasmosis, etc., from bats, so why would you need an intermediate to catch a coronavirus from a bat?
Also, what does that even mean, "Lack of a chain of mutations"? Of course we don't know the exact mutations that coronaviruses undergo in bats, because we don't have samples from every infected bat. Anyway, whatever that statement is supposed to mean, nothing about it suggests an engineered virus. Evidence of engineering would be contained in the virus nucleic acid sequences. If, for example, a coronavirus contained a gene from a completely different virus, that would be pretty obvious evidence of genetic engineering. Random mutations between different populations of coronaviruses only show that the process of evolution is still going on, as we expect.