After diving a bit into MRI's C code I've almost understood how this constant loading should actually work.
Marshal relies on `rb_path_to_class` and works like this: first it performs a check (`rb_const_defined`) to understand if the constant exists and it's not private, then tries to load it (`rb_const_get_at`).
Both operations look much alike, but the former returns booleans, while the latter raises errors or returns constants.
There are a couple more test cases not covered in RBX, I'll try to get them all in and implement them as much as I can.