If they were people without possibility of bias such as a large crowd at an organized fight which happened to not be recorded or taped, then their corroboration can be accepted with little question. When you have a bunch of people where there is possibility, not even necessarily proof of bias then there is doubt.
No but you couldn't verify anything you're saying. What may or may not have happened is not the issue. Evidence is the issue. Without it nothing an individual says means squat.
You would be justified in saying the person is not a fighter for there is no evidence.
There is no doubt as to who is a fighter and who is not based on evidence presented. There is no doubt about who is a good fighter based on the class of opponents they face. Again at best, if we assume, and it is an assumption, that Bruce did engage in unrecorded fights he still didn't fight anyone of worth. At best you can argue that he is a crap fighter.