They don't mention Sinnoh more than a few times throughout the entire game.
Agreed, though I think there's certainly more substance to an argument for DP remakes after the release of SM than before (though I'm sure the same case could be made for a Kanto return due to what was obviously going to be a very heavy Kanto-nodding game, being the 20th Anniversary and all).
• Type:Null/Silvally (and the latter's RKS System ability, as well as Multi-Attack) paying direct reference to Sinnoh and Arceus, in a number of ways.
• All Pokémon in Dimensional Research Lab referenced all being Sinnoh native (Bronzong, Palkia and Giratina) as well as the ongoing monitoring of space-time in the background.
• Something like 62% of Sinnoh native Pokémon not being available in Sun and Moon, with the only regions less represented being the Gen VI-present Kalos and Hoenn regions.
• The large majority of the plot of both Gen VI and now SM has been leading to something of a climax, with even the RS remakes suggesting to the player that what we know as the Pokémon world is actually happening in somewhat separate timelines (which I ultimately believe boils down to two, Mega Evolution and Non-, but that's besides the point), at differing times,somewhat strangely.
There's much that I've not touched on, such as the Linatone event near the Ruins of Conflict, but all in all I think the introduction of Ultra Beasts and Necrozma weighs it even more heavily in favour of DP remakes this generation at some point, and these hints seem about as strong, but not as on-the-nose, as previous hints to new games (remember, XY set up ORAS with the "40th and 45th anniversary" hints, and then SM with the backpacker who was in possession of the Strange Souvenir and first hinted at Alola; it's very possible SM could be setting up what we get in this generation and beyond also) beforehand.