The proposed API, MRB_API, is used in mruby. With enough popular support for that C API, it could force mainline ruby to modernize it's C API to that one.
Assuming MRB_API thread-safe-able. mruby is not thread safe at all, being a single-threaded script runner.
My expectation is that MRB_API can be implemented in a thread-safe way. The API manipulates the interpreter state (in the case of mruby, an `mrb_state`). The interpreter state itself and the implementation of the API can be thread safe or they can not be.
I don't think mruby even assumes the presence of threads on target systems, so it is completely reasonable that the implementation is not thread safe.
Assuming MRB_API thread-safe-able. mruby is not thread safe at all, being a single-threaded script runner.