Hello Frank,
acutally I did the research myself a few months ago. If you look up my previous tickets you'll find out, that I suggested the lines to be included in your docs.
Frankly, speaking I am quite dissatisfied with the developer support provided here. The last time I had to find the solution for myself, as well. Other extension providers (e.g. Akeeba) are capable of providing support on security aspects of their extensions. They give instructions what their extensions require, also in terms of htaccess.
In the case of SP Pagebuilder support, I have the feeling that there is not even an intention of trying to fix the issue together with the customer, rather the customer is asked to find the solution with someone else (this is not our responsibility).
Sorry, but my expectation is that the extension developer provides support for such security issues and what needs to be configured in htaccess. Especially in light of error messsages which provided.