MSG (Join to see) that's all true, great, and wonderful. But it is very hard to determine if SPC(P) (Join to see) knows or presents those qualities without actually interviewing the man, don't you think? If they gatekeep their promotions using unit lineage that is correctly given to the board, then how is the SM supposed to improve for next board?
Every board I've been part of begins with the members inspecting the Soldiers individually, their uniforms, their grooming, the placement of ribbons/badges/patches, etc. So it is fair to assume this SM had all that out of the way.
As for the SM venting his frustration in Social Media, I agree that perhaps he should not have tried that, but I would not go as far as to call it unprofessional. We use a lot of things under the umbrella of "professionalism" or "unprofessionalism".
This reminds me, would you call the video of the female Marine NCO crying and screaming in frustration after her admitted sexual harasser was allowed to return to work with her, unprofessional as well? At some point, frustration wells up and it naturally has to find a way to escape, in the modern world of Social Media, it is the natural venue for younger SMs to do so.

Hard to determine if he will make a good NCO if the board doesn't even interview him after correctly answering their gatekeeping question, don't you think?