Of course there’s more to it than that; but, it’s been long known that exercise helps curtail symptoms associated with anxiety and depression.
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.