Blue is in the Tackle Zone - so that is near the ball (law requires "close around the ball") - near enough for red to clear out the tackle zone (i.e. push Blue) or form a ruck.
You may say the law says this or that, but anyone entering the tackle zone (and we are only talking about the tackle zone here - not the 9-10 channel etc) - is in my book entering to either play the ball, or impact the playing of the ball. So they are liable to be pinged.
How about another scenario:
Blue get tackled, Red comes in to Jackle. The blue player who is running back now clears out the red jackler from the "wrong" side. He hasn't entered the tackle area through the gate, but he isn't playing the ball either, so according to the law it is play on? And until he binds on to the red player it isn't a ruck, so he hasn't entered the ruck from the wrong side either.
I'm PK'ing Blue for incorrect entry here. And that is entirely consistent with the potential PK in the original scenario - entering tackle zone from the wrong side (not through gate).