You speak with certainty which I don't really think is backed up by scientific fact, or quantifiable proof. Maybe you're right in full, maybe in part but maybe not at all.
Exponential curves start with very small numbers, but ramp up to big numbers. It was only a few weeks ago "we only have X cases" and "only Y people have died".
This virus spreads from person to person, with each person infecting, on average, c2.5 more people. When people spend time in confined spaces with others, breathing in the same air as each other, such as on packed trains, on busy concourses or waiting 5 deep in the bogs to have a piss, the likelihood of spread is greater, hence all the social distancing measures we have in place now.
So, of course, I have no proof that at a specific Albion game some weeks ago, the virus was spread, and I'm not even suggesting it was. However, on aggregate, the actions of the entire UK population at that time WAS spreading the virus which is why we are where we are now. Maybe a few people had it at the Palace game and spread it. Maybe they didn't. It doesn't matter really, as the overall spread from then to now is abundantly clear to everyone.