<<..if the professor says that something *is not inapplicable..>>

All depends what the meaning of "is" is, huh?

I'd say the phrase means, is applicable but with only a slight hedge and a big insinuation. When he doubles the negative it means it's the precendent is applicable, but the way he puts it make it, he wants to express some doubt -- but he's doing that tongue-halfway-in-cheek. He means it's applicable and he's so damn smart to know it, too!