In the early 1900’s, Rugby Union, or English Rugby, as it was known back then was the dominant sport in Canada
This statement is incorrect. The sport was not Rugby Union, it was Canadian Rugby Football, a much improved version of the game.
The game which Earl Grey's famous Cup was given to was a ball-control version of rugby which had no rucks, no mauls, no fair catches, no throw-ins and even no scrums. After a tackle, the teams got back on side and lined up with the forwards of the offensive team on the line of scrimmage. The middle three linked arms as if packing down, and the middle one heeled the ball back into play. There was no fair catch on kicks in general play, instead the offside members of the kicking team had to give the returner 5 yards until he gained possession. Kicks that went in-goal had to be run out, or the attacking team scored one point and the defending team had to kick-out from the 25 yard line. There was a 5-minute sin-bin rule for serious infractions too, but whether this was adopted from hockey into football or vice-versa I couldn't say.
These parallels mean that rugby league is to all intents and purposes an improved version of that original Canadian game. It's even possible that some of these innovations made their way into RL from Canada: Frank Cosentino's book Canadian Football: The Grey Cup Years documents at least one case of a former Northern Union player who played in the IRFU in those early days. Canadians like me who grew up with the CFL will instantly recognize the parrallels between RL and the so-called "Canadian" football we have here.