As far as I know (but I'm not the person to ask), their goal is to swap a new (Guile-based) Elisp implementation into Emacs that is 100% compatible with existin

As far as I know (but I'm not the person to ask), their goal is to swap a new (Guile-based) Elisp implementation into Emacs that is 100% compatible with existing programs. That seems like a sine qua non if their plan is to have any chance.