As long as you are using the word MEB correctly, yes, you aren't given a copy until the MEB is complete. When you get the NARSUM they will use it in their write-up so you need it to know if it was sufficient or you need to create additional documentation to support an accurate non-medical assessment. So the question comes to do you want/need to see it or are you just being overly cautious?
I don't can't think of any possible reason a PEBLO would have for not providing it early. It goes through the PEBLO, so there would be no reason why they wouldn't have a copy. Maybe the PEBLO sent along the only copy? Maybe the PEBLO is wondering why you're worried about it and doesn't want to do extra work? I imagine PEBLOs get asked for things all the time that don't matter so they'd get used to just telling people no. If you had a real need to see it, its information about you, I don't think they really aren't allowed to give it to you.
If you have a good reason to suspect the commander's letter is insufficient, I'd try to find other people you work with to write up a good assessment regardless of the actual contents. If you think it will likely be unimportant or in your favor, I'd probably just wait until the NARSUM is done and get it at the proper time.