I would doubt that he's her underling as well, and I would hope that he gets his own story. But, that doesn't mean that it's impossible for him to be in that raid. Plenty of raid bosses have had their own armies/allegiances, yet they've been in raids that are thematically different from themselves.
Mistress Sassz'ine and
Harjatan weren't working with the Legion in the Tomb of Sargeras.
Kargath and
Elisande were tenuous allies of the raid's main bosses, but they weren't the final bosses of the raids they were in.
Iskar didn't have much development concerning the Legion before Hellfire opened.
Tichondrius was sent to keep an eye on Gul'dan, and he was once the leader of the Nathrezim, yet he wasn't the end boss as well. Not to mention,
Gara'jal wasn't even working with the Mogu at that point (since he was trying to blow open the door there to get the secrets inside), and came back in spirit later on in Throne of Thunder.
Considering we already know that the story in Kul Tiras leads to Azshara (through Lady Ashvane), it's not impossible that Zul could have a tenuous alliance with Azshara as well, in order to push the Horde's story to her as well. As stated earlier, I
hope he gets his own, independent story, but it wouldn't surprise me if he didn't as well.