I'm a bit sceptical about this claim.That explains why magic isn't as prevalent on Oerth as on Toril.
Prompted by learning about Quests From the Infinite Staircase, I'm reading my copy of Beyond The Crystal Cave. The small fishing port of Sybar has a 5th level cleric and a 5th level MU among its inhabitants. And a magical garden in its hinterland.
The DMG City/Town encounter matrix, which presumably is suitable for use in GH cities and towns, has an indentured MU as part of each guards encounter, and an indentured cleric as part of each watch encounter.
There are plenty of MUs and clerics on the random NPC tables in the DMG too. The rulers of GH, according to the Gazetteer and boxed set, include plenty of high level magic-using PCs. The City of GH boxed set gives us a magical university, a wizard's guild and a Society of Magi as features of the place.
You can hardly throw a stone in the Great Kingdom without hitting a demon summoner or their summoned fiend.
Etc.
GH, as a setting, is permeated by magic.