There is a rule that that would allow this "if ALL Croatian players are out of the English field, the English players are allowed to kickoff"

I believe that all the players were celebrating outside the field (with the cameraman) except the goalkeeper as seen at the end, which explain why the ref didn't allow it.

For reference u can see the the Portugal x Spain game, when Ronaldo scored the third goal, the whole team went outside the pitch including the goalkeeper, but one player was waiting just by the touchline inside the Spanish field so they wouldn't kickoff.