This particular nudi has been spotted in singapore too. If its seen in singapore, its probably more widespread just not reported.
I've also read that, it has been seen since a few years but as we say in french "one swallow doesn't make a summer".
FYI it was also reported in Sangeang which has extremely similar grounds to Tulamben at the foot of a volcano. On the other hand it was never spotted in areas like Lembeh nor Ambon bay where the sand bottoms are screened everyday by hundreds of divers (at least for Lembeh) and eagle eyed guides (who sometimes found endemic species).
It's interesting because it raises two points :
Consistency .
I am not aware Pulau Hantu would attract divers from all around the planet to spot a specific nudi because the odds of seeing it are close to 100% like in Melasti or Kwanji.
There's a difference between spotting a nudi at some place and having it seen regularly in one place for years, as I remember diving Tulamben those 12 past years there always were donuts doto in Tulamben just like in Milne bay PNG.
Take Mola mola or thresher sharks for instance, they're all over the world though people would rather come to Bali or Malapascua to spot them because the odds and ease of seeing them are much important than anywhere else.
Distribution.
Seeing a specie in one place doesn't mean it is more widespread in its surroundings : Take for instance the case of the Banggai Cardinalfish seen in NW Bali that is normally endemic to Sulawesi.
The moment it populated Secret bay didn't mean it is spread out all around, because the reason of its presence there is very anecdotic.
Why is doto greenmayeri present in Milne bay and specific 2 or 3 spots in Tulamben (not all of Tulamben spots), rarely seen in Sangeang, then suddenly in Singapore .... no one knows, do you?
As you may know lots of people have been interested those 10 past years in nudibranch, tens of thousands of nudibranch tracking dives have been performed at very well known Indonesian dive spots, lots of books written, taxonomia of nudibranchs recalculated through 100's of new nudibranch findings, on the other hand I haven't heard/read of other spots where this specific nudi was seen while it is VERY EASY to spot it knowing that it lives on only one hydroid habitat and that it is not a seasonal nudi unlike some others, besides that if you spot one you will see more of 'em. They're not loners.
Have you heard about other sightings ? I haven't while I am reading lots about nudibranches.
If not I wouldn't dare saying It's widespread elsewhere, nothing more than speculation, which is the contrary of science.