Yes, Clx generates C++20.
The main motivation wasn't a particular C++20 feature, but using GCC, Clang and MSVC as a portable optimization and code generation backend instead of LLVM or custom machine code generation.
As for coroutines, no. Clx doesn't use C++20 coroutines because Lua coroutines are stackful, so the runtime uses platform-specific context switching instead.
My understanding of the original question is: what motivated you targeting C++20 instead of e.g. C++17
My bad ! There wasn't a strong reason to target C++20 specifically. I simply choose the latest standard available at the time as started experiments.
In practice, the code generator doesn't rely heavily on C++20-only features, so targeting C++17 would likely be possible with some adjustments.