I've read a lot of the PUA material and in my opinion it boils down to two things:
1) How to be a genuinely attractive person
The essence of being a genuinely attractive person is really nothing more than having a deep sense of purpose and genuinely pursuing it in all your behaviours. Being congruent with a purpose and not deviating from that to please others is really the core issue with attractiveness. It comes down to being reactive vs. being proactive. Proactive people are attractive, and PUA skills largely work to either imitate proactivity, or to truly become proactive. Some gurus would rather imitate ("fake it till you make it"), some simply teach personal development skills to truly take control of your life.
2) How to convey your attractive personality in an extremely short period of time
This is more closely related to the "pick up" art. There are specific skill sets that can be developed to pick people up that depend on the environment and the type of people involved. If you want to have the ability to pick up anyone at a certain place in a certain amount of time, that is of course something you can achieve through learning little tricks. However, you'll never get anywhere good if you don't also achieve #1.
So guys like Mystery teach both #1 and #2 to various degrees. People who are genuinely attractive tend to attract those people who get to know them but aren't neccessarily good at meeting new people or picking people up arbitrarily.
Some of the "PUA gurus" are very personal development oriented. David DeAngelo is pretty much just a self-help guru who is focused on a specific type of person. The socially awkward nervous guy. I think David DeAngelo actually uses PUA as sort of a hook to get people to work on personal growth. It's like how Christian proselytizers will hook you in with their tracts and stuff like that. Sort of a bait-and-switch for the greater good
That's my take on it anyway.