The guy is right in that no racer ever slows for a yellow flag (a point I have made for years), it simply does not happen at near all levels, as the rider that slows is the rider that will soon be passed (and that is from experience of flagging).
I would ask this guy to explain why Marquez deserved a penalty actually as if it is because he crashed under yellow, that is one thing but if it is for not slowing down, then all should be penalised. Even then, as for crashing under yellow it occurred because he (I suspect) shat himself a little knowing the penalty for overtaking under yellow and quite simply, hit the deck and Pol.
But I would also ask the racer, if the suggested penalty is for not slowing ..... pot, kettle?
IMO only but in this instance he does not deserve a penalty as the previous flag point was NOT displaying the yellow as per regulations and thus, MM had no idea until he entered the 'live yellow' point. As this is backed up by Pol's comments and reported elsewhere (alleged Race Control comments), then I would give Marquez a pass on this one as there was little he could do in reality as he likely was already positioned to take Pol.