I find minimal exercise dose studies super interesting. Of course you can train 30 minutes instead of an hour. What’s the worst that can happen? You are a bit slower in the end?
Just remember that 20-30-40 minutes on the bike is a million times better than nothing.
That will work to a small degree but you can’t keep lowering the time and upping the intensity. At some point, you are over threshold and doing VO2max workouts exclusively.