None of that is true. Not even close. It will work if there are laws with big teeth as the government can have contractors that scan for the header. Header missing? Adult content? User provided adult content even if it violates the rules? Well darn, there goes 10% of revenue. Also it is not "this is a porn site". RTA stands for restricted to adults. That can include literally anything that is not appropriate for small children including but not limited to social media, forums, gallery sites meant to be SFW but users content is not pre-moderated meaning people can see it before a moderator does. What Restricted to Adults ultimately means will depend oh the laws behind it.
An actually useful standard would need to be much more granular, and be designed as a whitelist not a blacklist, as I and previous commenters said.
[1]: https://wiki.whatwg.org/wiki/MetaExtensions#:~:text=Restrict...
Here [1] is the only external link that is relevant and don't take their banner out of context. This was originally by the adult entertainment industry to self label and self regulate but is now used on any site that is not appropriate for minors. Entertainment could be inappropriate humor for example. That does require porn. I would link to some inappropriate humor but that would be inappropriate.
"The RTA Label is free to use, voluntary, and universally available to any website that wishes to clearly and effectively label itself as being inappropriate for viewing by minors."