Debate fuels my Fire
I would put both as 'exist, but not common'. Gnomes in 5E are apparently not common either, but GH always had them as being abundant, so I'd change that....
I think the language is vague enough that you can decide either way (I actually think it is purposefully written that way), and I found it noteworthy that both were put in a separate category of looked at as more foreign than the other races.
Of course, GoS does make tieflings canon, there's an NPC of one (I think it fits well). Nothing in the book makes dragonborn so, but you could easily make them "exist but very rare."