Espruino on your watch!

Does this mean because nRF52 devices go to sleep - without calling setDeepSleep - automatically, so you have to have some kind in wakeup call to prevent the watchdog reset?

That's correct - auto mode will kick the watchdog each time around the idle loop - but on nRF52 it'll go to sleep automatically when not busy so won't get to kick it.

Even something like this should be enough:

E.enableWatchdog(5);
  setInterval("", 4000);