Or better yet, why can't the two projects be combined into one?

Wouldn't it be better to have two companies working on it instead of just one? Or are the implementation details too far apart?

Wondering same, as it shows full compatibility to cpython unlike Facebook implementation

My understanding is that can is only half the question. Yes, the changes CAN be upstreamed but my understanding (through others, so take it with a grain of salt), is that the CPython doesn’t like to take optimization changes that obscure the CPython code base. As a result, they often deny merging optimizations in favor of readability in the C codebase. Again, just my understanding and may not be 100% accurate.

Cinder is fully compatible with the CPython's C-API