is true to an extent, however only marginal. The only way to remove muscle mass is to overwork the muscle then starve it of recovery.
Muscles tear ofc and grow bigger when fed with protien, you have to do high reps, very high then remove protein from the recovery part, i.e 2 hours after a workout atleast.
i did this to great effect but you must be strict, verry strict and patient to get results, here is a link detailing it much better then i can. The article is for calves but the science is identical for any muscle group.
Btw the cardio you speak of will end up removing much more fat then you will probably like, weight training burns fat but equally pure cardio burns fat only much more, in my body experience anyway
http://www.calfreduction.net/content/train-reduce-muscle-sizei hope you achieve you goals