Nowhere in "Unification III" (DSC) did they say all Romulans relocated to Vulcan.
Spock's hope was that for one day the Vulcan and Romulan people would reunify. He said "It may take decades or centuries for them to reach it, but they will reach it". 800 years after that TNG two-parter was set, in DSC, we see the reunification had finally happened.
There's nothing wrong with that. In this case, "small universe" doesn't apply. It's a natural conclusion to an existing storyline.
The "Unification" two-parter itself was "small universe" if we're really going to go there. It was a 25th Anniversary episode linking TOS to TNG. So the "Unfication" episodes were in that territory already. I understand what Dukhat said. I just disagree with him in this case.
BTW, it would also make sense that Burnham would want to find out what happened to her foster-brother Spock and what he did with his life.
"Small Universe" is the crap we see in Lower Decks where shit is randomly thrown out for no reason at all as a gag. That's small universe. Not this. "Unfication III" was a continuation of an already-existing storyline. I have my own issues with "Unification III" but this wasn't one of them.