I use REBUG 4.65.2 in cobra mode and it for sure works. The thing is mmCM doesn't allow direct disc access, but, if you switch into multiMAN mode (via mM > Settings) then you can use the direct disc access feature. It'll auto start in mmCM mode next time you start if you're on cobra os.
If your on 4.70.1 then I don't think mM has been updated with the 4.70 pointers yet, that could be why, but, I haven't personally tested (I kinda thought it'd still work even without the pointers updated, but, I know some things don't work, this could be one; especially if it's giving you an error, typically, the option just wouldn't show up if your in mmCM mode). mM will probably be updated eventually if your patient, though it could be weeks.