There's a number of developers who get stuck on ROBLOX because they learned their creation tools when they were younger (they're easy to use and easily accessible to any desktop ROBLOX player), spent their formative years mastering their skills, and those skills turn out to be niche and not easily transferable to most other game engines. The choice is between basically restarting as a beginner in Unity, or continue making advanced creations on ROBLOX with all their friends and prestige they've earned in various sub-communities. To be honest I'm surprised it took this long for someone to try making an API-compatible alternative
Maybe the idea is that developers can release standalone versions of their Roblox games and escape the platform lock-in? Of course - whether their audience will come with them is a different question.
I have some kids into editing Roblox, and I'm a full time Unity / Unreal dev, and I would say that the Roblox editor and engine _are_ really good.
Kids don't care about fancy graphics, they care about connecting and running around with their friends, in a wide variety of games, that are downloaded and up and running within seconds.