That gives us an upper bound but the real answer could be anything between that and harmless.

So I mean "we don't know" as in "we don't have the complete answer".

But that's not how that works? If there is no statistically significant effect, then whatever effect there might be is so small that it's part of the background noise: we have a complete enough answer to say "there is nothing that can be attributed to just this thing". And we can say that with certainty because of the statistics.

From this specific accident. However, the evidence from radiation biology in general is that there should be a certain number of cancers (and no, don't feed me anti-LNT BS.)

But, you know, probably fewer than accounted for natural variations in the background level caused by different rock types in the area or exposure to residual fly ash from thermal power plants, right?

Why do you imagine that point has any relevance?

It seems fairly obvious that if the influence of a nuclear accident on cancer rates is dominated by other factors, one should look to mitigate those other factors before worrying about the nuclear accident as a contributor.

That's not how regulation works. One does not get a free pass to cause harm just because something else is causing more harm.