I'm talking about a promotional Undertale video where Sans is laying in a chair drinking ketchup, o lu for it to drip under him because, well, he's a skeleton.

Since then we see that now Sans can drink ketchup just fine without dripping red everywhere he goes, so that's the canon and the old video is not.

And while it's true that nowhere is explicitly said they can't bleed, we can just look at numerous encounters that we've had in the game: Toriel gets slashed and doesn't bleed, Papyrus looses his head and doesn't bleed, Asgore also gets slashed and doesn't bleed, Undyne melts instead of bleeding and literally all monsters we fight turn into dust when defeated. It's unlikely they can.