I commute solely by cycling every weekday (15 min each way, with some hills) and I’m still depressed. Must be more to the puzzle than that!
Due to life changes that are devastating, I’m walking/running 8-12 miles a day and swimming 3-5. I still have severe anxiety (along with medication and therapy), but the days where I cannot get to the gym or out are FAR worse than those that I do.
30 mins of biking probably isn’t enough exercise to get to the threshold required to reduce your levels. While everyone is different, I need to hit the 45 min mark to feel any sort of benefit.