I understand what you are saying Umberto.
I did not mean to imply that cardio is the only way to build Mitochondrial density.
The other way is through weight training like you suggested.

Fast Twitch IIB(Glycolytic Fibers) - are our power fibers and lifting in the 8-12 repetition range will develope Mitochondrial density

Fast Twitch IIA(Oxidative Fibers) - Lifting in the 12-25 repetition range will work for these.

Slow Twitch I - Density is best accomplished in the 50 and up range._

Strip sets and supersets get the job done as well.