The problem with the San’layn (I am delving into this subject for the first time here!) is that it’s not a race, it’s the name of the sect in the Scourge. The race would be Darkfallen aka Undead elves, albeit with a vampiric touch. What matters is your affiliation. It’s like saying that Sylvanas is Scourge because she once was a part of it. But she’s not anymore.
If you are an intelligent Undead, then you are free to choose what to do with your life/undeath next. Pledging loyalty to the Forsaken is a great way to be an official member of the Horde and the largest sentient undead community. It’s very versatile and canonically includes plenty of undead subtypes from Geists and Banshees to Wraiths, Death Knights and leper gnomes. So yes, DKs can also be part of the Forsaken.