You are absolutely correct, I thought I had heard this rule but clearly mistaken, having read various press releases I sincerely hope he gets a ban, it was a blatant dive.
Yes it does matter; the ref gave a penalty so saw the “offence” nothing was gained from the penalty so no retrospective action can be taken, in Hemeds case no official saw the offence but because it was seen by everyone else on TV he was given a retrospective ban.