Of course he was, he was an excellent engineer with specific knowledge of holodecks (in fact in that very episode he was called in to check out a problem with the holodeck).
He even said in the episode specifically to Moriarty that he couldn't exist off the holodeck.
In that episode they were trying to find a way to make it work though, because they had to. The idea with the transporters was to see if holodeck matter could be converted to normal matter which can exist off the holodeck.
Of course, eventually it would be possible for holodeck matter to exist off the holodeck, with the Doctor from Voyager. 126.96.36.199 15:37, August 20, 2011 (UTC)
Yes, it is true that with twenty-ninth-century technology, the Doctor became as unique as Data in having a mobile emitter. The Doctor was always "photonic" and not relatively substantive in nature, which is what Moriarty sought. ProfessorTrek 15:51, August 20, 2011 (UTC)