I would argue that league form is completely irrelevant.
It's a different competition altogether. Bayern as a club are hugely experienced, and that counts for a lot.
Arsenal have more than enough quality to win this competition, but they don't have any experience (recent or historic) to say they will.
In the 5 years that preceded our first victory we got to:
Quarters
Quarters
Quarters
Final
Semi
It took that much latter-stage experience to get over the line.
This is Arsenal's first Champions League campaign since 16/17. The first time they are playing in the quarters for 14 years. It would be an unbelievable achievement if they win the thing.