[kretaceous]

I don't understand so bear with me.

If the Uranium came from multiple supernovae, then why is it shocking that earth has different concentrations of U235? Moreover, how is it proof of a past fission reaction?

What if that "part" of U235 came from a separate supernova which is a little older and some more of its U235 had already decayed?

[ReaLNero]

There were unusual elements characteristic of the decay chain following a fission.

After a U-235 atom undergoes fission, one of the outcomes is it releases Barium and Krypton (and some neutrons), which then eventually decay to stable/semi-stable elements. If one of those stable elements is common in the deposit but otherwise rare naturally, it would point to a nuclear reaction having occurred.

Also note that the U-235 decay chain generally looks different from the decay chain following a fission reaction of U-235.