The point he is trying to make, not so effectively, is that Faeries from the start have more room for higher raw TPA/WPA numbers than a "Hybrid" because of 3/8 elites and not needing to maintain offense on top of a defense.
Say the Elf/Mage is trying to play a Hybrid with the basic 60 OPA and 45 DPA with his army out. He needs 10 elites per acre just for offense and 7.5 defspecs per acre just for defense. That leaves 7.5 pop/acre room for peasants, thieves, and wizards... combined. Of course buildings will improve his ME and lower those numbers somewhat, but Faeries can use buildings to raise their defense and reduce draft rates, too.
The point being that any hybrid elf will necessarily be incapable of fielding the same raw TPA/WPA as a Faery. Faeries simply don't have the pop concerns of running an offense and can raise defense with a 3/8 elite, which takes much less of their pop. Thus it is much more likely Faeries will be able to run higher raw WPA than elves...