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? If you tense up because you are afraid of intimacy, it's going to make connection harder. It's even worse if you engage in physical contact because you read on the internet that you should and then tense up because you are afraid of physical contact.

There are many sources for improving social image and attitude. It's happens frequently that people who start with PUA start to behave in a way that burns existing social connections.

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.

I didn't say "extra squeaky clean" I just said clean. Don't strawman.

A lot of woman openly state that they judge man by their shoes. Wearing leather shoes instead of sneakers isn't a high hanging fruit.

But much basic advice is freely available online

Much of that basic advice is given in a way to maximize bootcamp attendence.