A piece of digital text isn't a forgery; there is no concept of authenticity in it in the first place to serve as a backdrop legitimizing the use of the word "forgery". It's not like planting someone's hair, or imitating their signature. You need absolutely nothing from the victim, and to expend no effort.

The concept seems to have come from the original off the record (OTR) messaging proposal. The concern was that, say, a PGP signature could create a situation where there was objective proof that someone had created a particular message. By making forgery possible the theory was that someone could disclaim the message. I have been writing some PGP fandom articles lately which is why I am up on the subject. The relevant articles:

* https://articles.59.ca/doku.php?id=pgpfan:off_the_record

* https://articles.59.ca/doku.php?id=pgpfan:repudiability

So I am not disagreeing with what you said, it supports my contention that forgability is a silly cryptographic feature if there is no proof in the first place.