It reminds me there was a really strong bounty hunter team in my arena shard recently. Boba, Greedo, IG-88, Zam Wessel, Cad Bane. It totally destroyed my squad until I figured out the right approach. I bet very few of you have all those characters fully levelled, geared and modded, so you simply wouldn't know unless you had faced it. Was it better than my zMaul team in the end? Not quite, but it could easily have been and there are surely better possibilities out there still to discover.
How can you say there is a zMaul/sith META (or that there even is a META) when there are so many available characters and combinations and you haven't tried them? Most of us are lazy and/or cautious how we invest game resources, so we simply copy whatever arena team seems to be doing well to hold our ranking for a while without taking any risks. Luckily there are a few people who do experiment and think about squad options, and they discover great options for us to copy.
I'm almost certain that neither zMaul nor Rex squads are the best available in the game right now. Both of them are weak links in their respective typical squads. There are surely better choices and I can see that people are starting to experiment now with different ideas. And many of the players with zMaul/Rex squads are using them to hold in arena while they are preparing other, possibly better squads. Wait and see what comes next.