Most arguments that detail Jiren winning is based on many faulty assessments. The biggest one is his strength level and how is reportedly stronger than the G.O.Ds which is nebulous and incorrect in one important regard. Whis states that there is a mortal who is stronger than his god of destruction and this god was stronger than Beerus. Beerus objects to this and retorts to this claim by saying it was an arm wrestling match which he lost. I answered a previous question in regards to this and in that I stated that there are many variables that go into winning an arm wrestling match. Therefore, Whis believing him to be stronger than Beerus based on strength alone is too faulty and shouldn’t be used as a solid argument. Whis more than likely said that in jest.
With that said, it should be noted that all scaling on Jiren more or less started with that statement when they compare him to Belmod/Beerus. With many fans believing that since he is stronger than Belmod, that automatically means he far surpasses Beerus. However, I’ve already destroyed the baseline for believing Belmod is stronger than Beerus. Passed Whis’s jest, there isn’t anything that supports that.
As for Jiren, he fought and even landed blows on MUI Goku which means he’s relative to MUI Goku, I’m sure we all know this. Now, there isn’t an in-universe statement or display against Beerus to claim he did actually surpass Beerus. There is some scan that vaguely implies it but we’re looking absolutes, not vague implications. Beerus is still superior in terms of power until we get otherwise in-universe explantation or an official statement from real life clears this up. As of this answer, there isn’t one. Jiren caps out around MUI Goku.
As for Broly, his adaptability was unseen until the movie dropped. He was able to fight a zenkai boosted, 2x omen boosted, and MUI boosted, Blue Goku. Blue Goku is astronomically more powerful than he was during his first and second fight with Jiren. Broly handled and defeated Goku handly and forced him to fuse. This supports that he is at a bare minimum at Jiren’s level when he fought SSBKK Goku. This Blue Gogeta was vastly above and at lowest even with MUI Goku during the tournament. It’d make no narrative sense for this to not be the case. Broly was then savagely beaten and almost effortlessly killed by Blue Gogeta. That would support that SSB Gogeta is the strongest non god character in the main continuity.
Here’s where Broly wins, he was able to tank attacks head-on from this Blue Gogeta before he decided to get serious. This alone shows that if the fight goes on longer than it “should” then Broly would stomp Jiren out. Broly is too adaptable for a drawn-out fight. Now, of course, Broly wouldn’t start out at this level of power and Jiren would need to end the fight as soon as it begins or else he will lose.
Broly takes this more often than not since Jiren would not end the fight before things got dangerous.
Broly wins this 7 or 8 out of 10 times.