Is it correct to say that POST is not idempotent? Sometimes it is, and sometimes it isn't. Maybe it would be more accurate to say there's no guarantee that it's always idempotent.
POST is used to create resources that don't already have identifiers. So, if you're repeatedly submitting that form, your request is creating distinct resources on the other end. If the resource already exists -- if it has an identifier -- you'd use (an unsafe, but idempotent) PUT to replace it.
POST is not idempotent. If you fill out a form, submit it, hit back, and submit again, if the form uses POST, each submission may be processed separately.