I would bet on a joking intent: the guy was just using a more “technical” word instead of using the more common rouillé, which totally exists in French in this context.

Mon anglais est un peu rouillé car je ne l’ai pas beaucoup parlé depuis le collège.

The sentence above is perfectly acceptable.

Some might even go as far as to say that the teacher was intentionally using the wrong word to emphasize the fact that his English was indeed a bit rusty, because he wasn’t able to translate the expression properly. That would make it a subtle “double joke”.