However, the game itself is exactly the same as 'Lingo' - an old US show that still has a UK version airing right now. It has also had its own app for a while.
It's amazing how a couple of extra touches can make something explode in popularity.
If it had a single barrier (install an app, create an account, click through ads, etc), it would have been yet another of countless word games. It was a brilliant confluence for a momentary explosion in popularity.
All along, though, people were yipping about the grand benevolence and moral supremacy of this version versus clones (when the app itself was, as you mentioned, not that derived from an existing game, even aping the coloring), and that all looks pretty silly now that the creator quite rightly managed a pretty lucrative "exit" for a trivial work. And I applaud them for it, and respect the brilliant choices made to get there.
It was pretty popular in the Netherlands as well: https://www.youtube.com/watch?v=9959YYGuEBM&t=45s
In the Netherlands it started with 5 letter words, as in Wordle, and later they moved on to 6 letter words for variety. But the principle remains the same.
Part of its success is the simplicity I think, and the fact you don't need an app.