They don't need to be tied, but CDPR made their release dependent on each other. I assume it's simply the effort issue. I.e. if they are to release the engine, releasing the editor is a requirement anyway, and developing it will be funded by those who pay for the engine. And if they don't release the engine, releasing the editor alone will put on them the burden of supporting and developing it with no profit. So I guess they don't want to do the later. Apparently CDPR don't care about modding alone.REDengine and REDkit aren't mutually exclusive.
You're talking about a whole game engine and on the other hand tools that let you modify a specific game that uses it.