Actually, because magen -- a word that's really supposed to be a shortcut for gens magica -- is coincidentally written the same way than Magen -- german for belly.
It's the same joke -- somewhat -- that these critters from Planescape whose name is Busen -- bosom, in German.
PS: If my German isn't too rusty, what Knight wrote would mean something roughly akin to:
"I havn't tried, but I don't know if my stomach would tolerate it."