I think it's because the SoC hardware is made by Rockchip.

By the time Rocknix forked JELOS, it was available for far more than just Rocknix chips. I think it's a pun on immutable distros being set in stone, but I could be making that up. You can ask in the Discord if anyone is really that curious.

So they finally found a use case for those CDs made out of rock.

I totally forgot about RockBox until I saw this headline, like yourself. That kept my first-generation iPod Video running far longer than I would have expected.