I agree Ian. I agree too with the OP that's the thin end of the wedge. The easy way is not to blow the whistle for the kickoff, remind the kicker it is his responsibility to ensure his teammates are onside. If, as in this case he had started play, little option but to blow up the offense.
If Nigel,Owens, who is well paid for reff'ing this match. With the benefit of fit professional players who know the LoTG and what is expected of them at all stages of the game. (Who are frigging paid for playing the sport! Offside at the kickoff is professional negligence for these guys.) With the support of two fit and knowledgeable assistant referees running touch for him, along with a TMO safety net to catch any offenses that slip thru. If he is going to let the basics slide, not only because he feels he has bigger fish to fry with professionalism/fouls creeping into the Elite game, but also because he knows he has the on-field presence to assert his authority when necessary. Then he is letting down every wouldbe ref like myself, who lacks all those advantages, but will be faced by players convinced that to let those things slid is a sign of good reffin'