No sealing ducts would be one of the worst things you could do with duck tape. It would only last a short time.
It was originally used in WWII and was called “duck tape”. I doubt if they used polyethylene back then but I don’t guess I know when poly was first used.
I grew up in the construction business, and have been working in it myself for almost forty years. Trust me - the tape we're referring to, was most definitely used for sealing sections of a/c ducts together for decades.
It's called duct tape for that reason.