Yes. Any witness testimony is evidence/proof. Jury takes all the evidence and decides what it thinks happened, and applying those facts reaches a verdict based on the law.
If the two off duty policemen testify that basically they were assaulted without provocation, that is sufficient evidence to convict if the jury believes them (beyond a reasonable doubt of course, which is a high standard). You don't need videotape or corroboration to convict, although if I was prosecuting the case it would be nice to have.
Of course if McCoy et al. testify that the cops spit in their face and that provoked what happened, that is evidence too. The jury decides what is true and not true.
At this point, there is very little here to illuminate who was at fault or what happened. It is all conjecture. I mean the cops have incentives to lie, e.g. not getting fired.