Actually, I think it’s somewhat possible.
It’s possible to be a genius (in mathematics, for example) and not be good with something like pronunciation. The problem is, it’s not just this one incident. It’s the overwhelming evidence from numerous incidents. It’s also the overwhelming lack of any actual accomplishments in his life. EVER. The lack of any transcripts, writing samples, SAT scores, LSAT scores, college admissions records, or anything else that would show even a modicum of intellect. The only writing example that I can recall seeing was a super lame attempt at poetry. (No, he did NOT write his books. They were ghost written. But I’m sure you already knew that!)
Bonus: Mispronouncing corpsman highlights the fact that he is totally out of his element with all things military.
All the writing samples that are verifiably from the pen of Soetoro (i.e., not written by Bill Ayers) have one thing in common:
He CANNOT get nouns and verbs to agree in number.
A sure sign of average IQ, at best.