Most physiologists believe that each individual have a set fiber distribution, but that you can change the characteristics of the fibers through training. Basicly there's slow twitch and two kinds of fast twitch fibers.
The quickest fast twitch fibers doesn't have many, if any, myoglobin, capillaries or oxidative enzymes, meaning they have to produce energy without oxygen. That's anaerobic work, from the Greek an (without), aer (air) and bios (life), meaning something that lives without air.
Usually when you do plyos and other high intensity stuff, you are doing anerobic work and cultivating both your fast twitch characteristics and anerobic energy systems efficiency. I guess his thought process is that a mask will further restrict oxygen intake and add to that process.
I haven't looked into if there's any scientific evidence for that, but personally I don't think the mask will work that way. Just like the mask won't work like high altitude.