Yeah they originally had a separate branch "Rider Unreal" but now it's part of Rider, C++ support seems mostly the same across both, except for project support. If IIRC Rider doesn't support CMake projects, but it does support vsproj projects, so if you generate vs files from cmake it'll load fine, but probably better to use CLion for any non-UE projects if using C++. I guess they're seeing it as .NET/GameDev IDE.
Honestly I don't know why there are so many almost identical IDEs.
Honestly I don't know why there are so many almost identical IDEs.