I believe you are using the "motte and bailey" analogy incorrectly here. It is not supposed to mean "the worst thing you can associate with a group", but rather something like "a central teaching which is rather indefensible in a debate, but they keep returning to it as long as there is no opponent in sight".

Normally when people say "PUA techniques," they mean something narrower than "anything you might do to increase your attractiveness."

Yes, they mean "behaving like (a PUA model of) alpha male". That includes a lot of things; and different people put emphasis on different subsets. The original approaches had a lot of "hacks to fool the alpha-male detector", and negging was one of those hacks. (And it was not meant to be used on every woman all the time, but as a way to get attention of a woman way above the man's level.) The recent approaches are about "just become the alpha male, duh".