APT standards for advanced persistent threats, that is, describe actual threats. Depending on who you are talking to and the context, there are sophisticated and targeted attacks from advanced players with a lot of money, such as states or criminal organizations – or just whatever is more complex than firewalls and basic antivirus could detect (as in "it was n it's our fault since it was an APT attack" when you did not care enough to secure the network).
ATP is a marketing term of the "next generation firewall" type, which should highlight the ability of an analytics system to detect (prevent) and prevent threats from "traditional" systems. Expectations are that it will not only detect "average" but "more" attacks. What exactly is this "plus" and the "best" is not clearly defined, but it often involves many buzzwords, such as the artificial use of information to detect threats or information about threats in the cloud.
The similarity in the abbreviation between APT and ATP may be accidental, but associations created by the consumer are probably welcome by the seller's marketing department. But only a few vendors explicitly state that ATP is actually capable of detecting APT.