Most of these were trained/tested using third world labor on mechanical turk. It actually helps to approach the problem from that understanding:

"Would a person who speaks English as a second language and who is getting paid less than a penny to decide, call this a motorcycle?"

Dude needs to answer about a million of those to make money, so he's not overthinking it, your knee-jerk answer is probably right.

My big interest right now is motorcycles yet I still recently failed a motorcycle captcha

I find that whatever your snap judgment is will be the correct answer, so basically yes the moped usually counts as a motorcycle