I've watched the incident.
This was not a case of something being allowed but rather a case of of something being missed. Throughout the game Matt Carley had been fastidious about holding back the defensive side at a lineout. He used one horizontal arm to hold them back and some verbal communication. He didn't pay particular attention to the attacking side. Was this the role of the AR's? Who knows, we were not at the pre-match briefing.
At the particular incident in question the referee was not facing the attacking side but was looking towards the defending side who had started to creep up immediately the maul was formed. He had his back to the attacking side so missed the extra players joining. The AR is seen to be very focused on the blind side of the maul/narrow side. He also was partially turned away from the attacking side so at best might have seen some advancement in peripheral vision. Should it have been a call from the far side AR? Certainly an incident that should have been raised during debrief.
When the maul was called over only one extra player had arrived. The second arrived a second or two afterward but had crept up offside before then.