Yes; I think the crux is that a properly capable rich-text widget tends to be neither fast nor light, really, so is a poor fit for our intent.
There are a variety of options out there, even some that are “stand alone” and could be integrated with fltk if you wanted to go that way, but if you have a serious need for rich-text handling, there are other toolkits that are probably more closely aligned with that sort of space... Horses for courses, as the saying goes.