The early Strongholds were all arranged directly by the System – neither the NPCs nor the monsters had anything to do with it.
If the System drew out a piece of land in the Plaguelands for a Stronghold and the players occupied it, the NPCs and monsters in the area would not attack it, no matter how evil they were. Accordingly, it didn't matter if you were Alliance or Horde, as they wouldn't defend for you either.
However, for Lu Li, things were different. This was an NPC he knew from a Story Quest.
To put it differently, he had helped the NPC at a crucial moment and had now established an offensive and defensive alliance.
Once they had captured a Stronghold in the Plaguelands, then Tirion Fordring would send Tyr Hand's guards to help them defend it. Of course, for the sake of balance in the game, these guards were not that strong and there were not too many of them.
