Yup, cycling is usually only better if the alternative is to do nothing. Most activities i.e. running, swimming, jump rope, shadow boxing are better. Heck, almost anything's better than cycling.....fencing, ultimate frisbee, ***, football, soccer, skateboarding, rollerblading, badminton, tennis...haha....you name it.
Carl Froch never runs, instead he gets out on the bike. So if it's good enough for one of the fittest boxers around I'd defo say yes.
Exercise improves fitness proportionally to how much muscle mass it forces you to recruit. Running is better than cycling, cycling is better than swimming in that regard. A benefit of cycling is that you maintain strength/muscle gains better than with running, and it is also low impact and easier to recover from than running. That allows you to focus more on boxing fitness. For complimentary conditioning cycling (taking these factors into account) is better than running. Running is better if you want to lose weight and you're in a running sport.
