I also think that you are misinterpreting what the advice is. It is not that you should mislead other people about your skill. It's that when someone wants to pay you to do something and they are aware of your expertise, you should take them up on it.
If you've read a lot and written about lean start up methodologies, but have not yet tried to start a business yourself, and someone come to you, aware of your level of experience, and asks to pay you to come and look at their business model and take them through your reactions and feedback, you shouldn't refuse just because you have no direct experience. They know what they're getting, and they want to pay you for it? Do it.
If you've never assisted at a wedding, but have posted some nice pictures from your phone on Instagram, and someone wants to pay you to take photos at their wedding, you should do it. You should make it completely clear to them what your experience level is, and that they probably should hire a professional, but if they still say they want to hire you, do it.
But of course the e-commerce site is a different story. If you actually don't know anything about basic web app security, and somehow you know that you don't know anything about it and that it's important (we are talking about approximately four people worldwide), then no, you shouldn't build an e-commerce site without insisting that they also hire someone with security expertise.
>But IMO too many people are straight-up lying about what they are and what they know.
There are a fair number of those yes, but I suspect that the majority of people who seem to be doing that are really just completely ignorant of their ignorance.