@AngryRhombus563 Strange, when I was a teen on the internet it was referred to as cyber sex, which was the digital equivalent, but times do change. Regardless, if it has been in the JP TOS since 2012 (last time it was changed based on the page's date) and a support ticket said it is a breach of the TOS, then it sounds like they aren't the ones trying to twist the definition but rather others.
If you also pair that Japanese TOS with the first Global TOS (no longer available, but screenshots still exist around the forums) you can read that it starts with Post, upload, publish, submit, or transmit before listing each term. That is a blanket way of saying in no way, shape or form is any of this allowed in the service. The area of interest is (iv) in regards to "ERP".
Though it doesn't really matter that someone specifically asked "that also means in private chat?" since if it is stated that you agree not to do any of the following in connection with the game service, they aren't gonna turn around and say "but in these circumstances you can".
Lots of terms will be included in any legal document for the purpose of protecting the business even if they aren't really what they are concerned with. I would bet my own account that no one has been terminated based solely on the grounds of a consensual ERP session in private that absolutely no one else had any possible way of seeing.
If individuals want to be paranoid about it on an individual level, that's fine and dandy, but I find it very disingenuous to speak that it is being used actively for termination without concrete evidence, which none of us will ever get anyways.