But Broly is still the strongest too. Both in and out of universe sources confirm it. And since Gogeta Blue is stronger than Broly, logically Vegetto Blue is stronger than Beerus too.
You phrase that as if Vegetto if formed during DBS Broly would be the same strength as he was in the Zamasu saga, as if Goku and Beheta haven't changed their power levels since then.
DBS:B Gogeta is formed from stronger components than Zamasu saga Vegetto. DBS: Broly Gogeta is stronger than prior fusions. Therefore you can't make a claim to what Zamasu Saga Vegetto's strength would be based on DBS:B Gogeta because the root of their powers are not the same.
Are you one of those people that think Goku and Vegeta hardly change their power levels aside from gaining new forms past resurrection F?
Broly could have risen past the Granolah saga powers from training since that saga ended.
It is possible that Broly wasn't factored into the wishes to be the strongest if he was at Beerus' planet since that is outside of the universe. I'd rather have more direct confirmation on that.
Both the anime and manga have given erroneous comparisons to Beerus. Gowasu said SSJ2 Goku rivals a god of destruction, yet GoDs are still considered a tier above everyone in the ToP.
The writers just don't know how strong they want Beerus to be and make these kinds of statements to stir up hype and discussion. It works since we are here discussing it.
As far as I'm concerned Beerus is still leagues stronger than everyone and prior fusion characters are already surpassed even by the Granolah saga, and those characters are still weaker than Beerus.