Dear secretagent,
Oh, no one claimed it was a proof. The historians would call it a reasonable conjecture. If you like it, fine, if not, not. Considering that it’s pretty much agreed upon that these tosfos were written towards the tail end of the tosafist period, this tosfos fits neatly into the pattern. But proof? No.
To mobico,
We have no verifiable perushim of Rabeinu Gershom. In that period, anonymity was the rule. It was written by Chachmei Magentza, a loosely defined group in the period of Rabeinu Gershom and afterwards for a generation or two. Their perushim were rendered obsolete by the greatness of Rashi’s perush.