Could it not be that he was speaking from first hand evidence of the people he was locked up with?
Also labelling criminals from Romania as Romanian is simply a statement of fact. If they were Japanese Yakuza or Chinese Triads I would expect him to call them Japanese and Chinese respectively.
All Romanians != Subset of Romanians in UK prisons.
He also mentions gangsters. It's quite within the bounds of what he's written that Romanian criminals in UK prisons that aren't gang members might also not want to associate too closely with the gangs.