Okay so you know what’s a thing that occurred to me?
Look at Blaytz, and look at the merfolk from s2e2. Look at his complexion, markings, head fins- he even has little antennae like several of them do!
You know how the mermaid planet is never named?
What if it’s Nalquod- Blaytz’s planet?
Think about it. Blaytz would really flit in perfectly with the merfolk. The only real difference is he doesn’t have a tail, but, even though Blaytz is obviously a sea creature, we only see him on land. And we only see the merfolk underwater- though they make it very clear that being on land, and on spaceships, is something they can do.
And the merfolk take a lot from human lore of mermaids- which very often features the idea that they’re able to grow legs and walk on land.
Several things in line with this:
The bubble coral makes perfect sense. Nalquod was part of a coalition of five planets. In its heyday, it would have needed to accommodate air-breathing guests. Frequently.
Luxia has creatures large enough to transport Voltron Lions.
The rebels immediately latch onto Lance as their savior explicitly because of the Blue Lion. Even though Hunk also has a Lion, they’re not nearly as concerned with him. It’s specifically the Blue Lion that they recognize- Blaytz’s Lion. The same Lion Luxia basically doesn’t even bat an eye at waking up in its mouth but immediately trusts and tries to appeal to the people there.
We have no idea how old Luxia is or how long she’s been leader. The mermaid city appears very well-established. Plaxum states that Luxia froze the ocean over by blocking the “thermal vents” and then moved everyone down to the depths, and Luxia talks about opening contact with other planets “again”, telling us they once were a spacefaring people, and also, that unlike many things that ultimately Luxia wasn’t responsible for, the Baku was- Luxia froze the ocean before the Baku took control of her. Plaxum, not knowing about the Baku, might conflate the two because Plaxum is probably much younger- she’s noticeably smaller than Luxia and Florona, after all.
So why would she do that? Why presumably call all her people back from space, freeze the entire surface of her planet, and hunker in the depths?
Well… the comics show us that some of Altea’s old allies are still alive. And again, we don’t know the lifespan of a mermaid.
If Luxia is over ten thousand years old- the Blue Lion is a nurturer, and Lance did shove Coran out of the way of a bomb.
It’s possible Blaytz was unable to escape himself, but, he was able to warn his homeland that Zarkon was going to attack them. And Luxia, next in line- Blaytz’s sister, daughter, niece, something else- locked down the planet so by the time the fleet got there, they found an icy, lifeless wasteland- and may have assumed that Nalquod’s people had fled the planet.