That's BS.
These mentors that would like to teach than to make money for themselves, they're only in it because they can easily make money from those newbies.
I agree to just self study but maybe some of their personal experience and teachings could be of useful for someone who aspires to become a full time trader.
Not all of them are just concerned about themselves and the intention to just milk newbies because some of them are dedicated to their role in helping newcomers.
Choosing the right mentor for us is quite difficult. That is why it is encouraged to just do it on our own. The availability of information online makes it easy for us to learn. But it is a different experience if we have a mentor who guides us, as I believe it was fast and easy for us to understand, especially on technical issues.
I won't say it is not necessary, but if we can afford to acquire a mentor, it is a good idea. Provided that it is reliable and can be trusted.
Expect for the very few that you've mentioned.
You're right that a newbie needs to find a respectable mentor that really is in it for the teachings. I've seen a lot of courses paid and free and there are also those that are experienced that don't ask for any money.
So, if a newbie is out and wanting to find a good mentor. They can easily know that if every material is given, they're asked to pay.