Nowhere in Sylvanas' statement is that distinction made between Alliance-aligned undead and Horde-aligned undead. All she said is that the Alliance shouldn't deny the Forsaken's rights to Lordaeron, because they died for that land. That's it. There is no distinction made whatsoever.
As a reminder, Sylvanas initially wanted to join the Alliance. It's not like she sees the Forsaken in the Alliance as some kind of sin or crime. It was opportunity alone that led them in the Horde.
The Forsaken are not defined by their membership in the Horde, but by their past as fallen Lordaeron defenders.
That is what gives them a claim to Lordaeron.
Therefore, the Horde should not deny the Alliance Forsaken's rights to Lordaeron.
