Does cardio earn muscle XP?

Yes, some. An activity earns XP by the minute, and a share of that XP lands on the muscles the activity uses.

A walk puts most of it on your quads and calves. A run works the same way, with more XP per minute.

The share is small next to a session aimed at that muscle. Cardio tops a muscle up. It does not replace training it.

Learn more: How much XP does an activity earn?