Bayern have been lacklustre all season that’s true, they do have players in their squad who have won big trophies though. They have won trophies playing in high pressure games.
Arsenal are an exciting young team but they are yet to prove they have the balls to win when it really matters. They are surely capable but until they prove it, you can’t rely on them.
All this is true, however, the problem is that the bet must be made before someone has proven something

For me, this pairing also looks incomprehensible and random (especially because of the playing styles), but if you look only at the facts, Arsenal are much more stable this season and are generally stronger than Bayern. On paper this is how it will be in reality, we will see very soon, but it seems to me that it makes sense to believe the bookmakers. By the way, if you look at brands and cash flow, Bayern definitely looks cooler, which means the bet on Arsenal is even more justified.