Right but reloading a static html asset (the reply iframe consisting of just a textbox) is basically free. So long as you separate the chat history window and the reply textbox/form into two separate frames, I don’t see the problem.
Using JavaScript is also “basically free”.
There’s no problem with either listed approach, GP just seems to consider a method without page reloads as adhering to a harder constraint.
Using JS would have broken the threat model of the dark web chatroom. So it is a non starter in that context.