Whis says that the rumored mortal comes from a universe where the GoD is stronger than Beerus. Then Beerus counters that he hasn't fought that GoD and merely lost in an arm wrestling match.
Anime is building up Jiren to be that mortal (but still haven't confirmed/outright stated it. Though Whis said something along the lines of "It appears the rumors are true" but that's not a definitive statement); so if it's Jiren, that would make Belmod stronger than Beerus (at least in an arm wrestling match)
In the manga, Beerus outright stated in an argument with Quitela that he only lost an arm wrestling match to him and that a fight would be different