While it does sever the Forsaken’s territory in the Eastern Kingdoms and isolates Silvermoon, the Horde still controls the northern reaches. They hold Andorhal and while the Plaguelands is by no means safe to traverse, there’s no real outlet for Alliance assets to deploy by any means other than directly through Andorhal.
I wouldn’t say it leaves them ‘dangerously’ isolated, the enemy has one hell of a march to cut through territory to reach Silvermoon. It’s gone from the northern reaches and the center with access to a coastline from Gilnean ports in Silverpine, to just the northern reaches.
I’m still a little confused as how the Defilers lost Stromgarde, they had every advantage, but whatever? BFA was a statistical nightmare that didn’t make sense to begin with.