United cards are from Chase and they are the 5/24 rule apply to them. Chances are those may be simply invitation to apply, as I don't recall Chase sending actual preapproved offers of co-branded cards. I recently got a Southwest card invitation and I can clearly tell it's clearly a invitation to apply and not a prequal. If your APR given in the invitation is not a fixed value and instead says "x% to x% based on your creditworthiness", and the mail does not have a prescreened opt-out notice, then it's unlikely to be a preapproval.
