The Absurdly Optimized Pancake: Leavening Chemistry, Acid-Base Stoichiometry, and an Interactive Calculator
(www.absurdlyoptimized.com)
I'm geussing they mean custard in the same way as a properly cooked French cruller. The center of the choux dough is cooked but the mouth feel is very custsrdy. I haven't had a pancake like that, but it sounds good.