The shattering dimensions feat is extremely inconsistent with their manga capabilities, unless you wanna argue that the fusion dance is an infinite increase, or that Broly got infinitely stronger after Gogeta appeared. They never used CSSJB in the movie, while we see them likely using it in the one panel we get of the manga version of the fight, where we also see both Goku and Vegeta fighting together before they should be, since they were still in the ice area. Goku remembers Ribrianne at the start of the movie as if she was a noteworthy fighter, even though he literally didn't even see her in the manga, let alone fight with or praise her, she just fought 18 for a bit, was weaker than her, then was eliminated by the invisible fighter. Whereas in the anime, Ribrianne was somewhat relevant, and fought with Goku, impressing him to some extent. Vegeta also noted how much stronger Frieza got, but in the manga there was no implications of a significant difference, just that he got slightly more powerful, contrary to the anime where he was much more powerful than in RoF. In the one panel we see of Frieza in the manga version of the Broly movie, we also see him in his first form, even though nothing implies he's used that since RoF in the anime and movie versions of DBS. So no, it's not really a baseless assumption at all. Just because they all fought together in a battle royale doesn't mean they'd be even in one on one fights, with Beerus being directly implied as one of the stronger ones on multiple occasions. Actually, in the manga, it was only said that Jiren has a higher level of raw power, and Toppo felt that it was worth specifying that, which still implies that Belmod could be superior in combat overall, but has a lower level of pure strength, and again, no actual confirmation of Belmod being on par with or superior to Beerus. Actually there is plenty supporting my claim of it, including the fact that it's what most people would assume unless they're directly trying to deny it. There's much more evidence supporting the actual movie specifically being for the anime than there is supporting otherwise, and taking Goku's word as concrete evidence, especially when it's not a direct statement, isn't very reliablable.