This is a bit tricky to answer. Low-level, steady-state aerobic training will atrophy muscle, so it can be true. But “cardio” hasn’t meant aerobic zone training since Richard Simmons’ heyday in the 80s. Cardio is a catchall term for any training that elevates your heart rate for the entire workout.
These days, since almost all weight training is done circuit style, your heart rate remains elevated during both cardio and weight training workouts. Modern cardio training is almost always an offshoot of interval training, which means it’s a mix of aerobic and anaerobic training. And this builds muscle.
Probably the easiest example to help you understand this is to look at some of the Beachbody® Success Stories. Take a look at the results from INSANITY® and those from P90X®. INSANITY is a “cardio” program that uses no equipment. P90X, on the other hand, requires you to lift weights every other day. Yet, overall, the results you’ll see from the two programs are remarkably similar. People tend to both lose weight and gain muscle. Adding muscle increases your metabolism and that’s what helps you lose weight . . . as long as you’re not eating too much. Now with that said, you’ll need to take into account your body type. Some people like me who are naturally skinny turn into a bean pole if committing to just a cardio regimen. Others tend to bulk up rather quickly if just committing to an anaerobic regimen.