No prepayment penalty, see https://www.americanexpress.com/us/loans/personal-loans/
How about saving on interest of the months you paid it off early? (checking your link now as I ask this question)
I don't believe the interest is front-loaded on these loans but you'd have to confirm the details in the loan agreement.
I gotta say this : I still don't know about my interest dilemma, but that had to be the EASIEST loan I ever did. Submitted the app yesterday late afternoon, I was approved immediately, and then this morning I woke up early and the money was already sitting in my account. For this and many other reasons is why I love being with AMEX!
I've had a couple of 'em, and my experience was the same as yours... easy-peasy.
About your interest question... I paid off my last one early, and there was no "refund," per se, but the savings comes from not having to pay interest on a zero balance. Like @pizzadude mentioned it's a no-fee/simple interest loan, so there's nothing to 'refund'.
I suspect they use that boilerplate language to give themselves room if they ever make changes to the program, but at this time it's moot.