Yep. Ability to paste file paths (without invoking some other key sequence first) into the gtkfilechooserwidget.c remains broken in Gtk3 and Gtk4. Broken in the sense that you'll get an error message. They say Gtk3.x isn't frozen but it is when it comes to the gtkfilechooserwidget.c. They won't work on it and they won't accept fixes from outside sources.

AFAIK GIMP does not use GTK's builtin file chooser, specifically because it lacked image thumbnails up until recently.