As far as I (and seemingly most riders in Spain) have understood this, this actually came into effect on 1st Jan this year. However, it also seems that, as usual, having not thought it through in the first place, a free-for-all has ensued and they have now added the requirement for it to be permitted by signage. The speed limit has always been there.
My feeling is that in almost every sensible case, if traffic is backed up and there is no signage to permit this, no traffic cop is really going to turn their blues & twos on and go chasing after a biker or bikers through slow or stopped traffic (because there won't be just one or two bikes doing it). At most they might park their patrol car on the hard shoulder to dissuade them, but that would be petty and pointless and, really, they have better things to do, like sit in their warm, or cool, car and wile away their shift without unnecessary complications.
A caveat to this might be to (sensibly) avoid doing it in front of a traffic cop who is out of their car directing or stopping traffic unless it's permitted.
The idiots this is designed to dissuade are those who just overtake on the HS in moving traffic because they are impatient, even crossing slip-roads. I've seen quite a few doing that lately!