If you read David Burns "The Feel Good handbook" he makes the point that showing vunerability is a condition to get someone to love you. A lot of behavior that a lot of beginning PUA's adopt go in the other direction and might make it less likely that the person get's into a long term relationship.
It's a balancing act. Most people are a lot more likely to show too much vulnerability as opposed to too little, so the advice to appear less vulnerable would seem to be justified. Similarly, a lot of the things PUAs say "don't matter" actually do matter, but only as a last resort. It's silly to put a lot of extra effort into things like making your flat extra squeaky clean, when you can pick the low hanging fruit of improvements in your social image and attitude.
And if PUA always involved spending "large sums of money" on bootcamps or proprietary material, you'd be quite right - it wouldn't be nearly as interesting as it is. But much basic advice is freely available online, although it may require some time and effort to find the best communities. (And that's one reason why I think one should be aware of the label, despite its problems: it's an easy way to find interesting material.)
Most people are a lot more likely to show too much vulnerability as opposed to too little, so the advice to appear less vulnerable would seem to be justified
I don't think that's true. Openly and directly speaking about one's desires for example isn't an easy skill. Many guys are tense because they are afraid to fail or to be rejected and put up a lot of barriers towards genuine intimacy.
It's also worth noting that you speak about "appearing vulnerable" while I speak about vulnerability.
If a woman touches you, do you tense up or do you relax? ...
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