I think he was joking.

With more and more RNAV departures being published it is an interesting question but the same problems would occur on a conventional SID if the primary navaid fails which I suspect is just as likely as the failure described above. You need, as always, some sort of plan B before you stand the levers up.

Jokes aside, AP on in this case is quite a good idea, as it will be capable to fly P-RNAV departures, even if not authorized... On the other hand, deviating from the SOP is
absolutely prohibited because doing so
means an immediate smoking hole in the ground. So better fly in the valleys manually guessing the headings because no FDs mean no P-RNAV... :-)