It's already been shown in Warcraft that younger races learn much faster.
Humans like Jaina, Medivh, Antonidas managed to reach a level of magic that rivalled the likes of Kael and Anasterian despite the latter 2 being centuries older.
Arthas could duel Illidan despite Illidan having much more experience.
Hamuul Runetotem became an Archdruid in just a few years whereas plenty Night Elves have thousands of years and experience and aren't there.
So saying Malfurion studied for longer is pointless, Thrall/mortals in general just learn faster.
Malfurion is also strong because the spirits/Elune/whatever else controls nature likes him. Both Druids and Shaman are just servants of their respective 'gods'/deities.