70 wasn't a terrible score, and Rory didn't play badly. If both Cams shoot good scores of 67, Rory wins. But then again, if Hovland shot a 67, he would've won.
St. Andrews isn't hard to score at these days, and while Rory didn't have a massive chasing pack right behind, anyone close going really low could be a problem (someone like Dustin Johnson shooting a 64 would've matched Rory's 70). While you'd normally not be fretting about people shooting 64 or 30 on a back nine, conditions this week were primed for that possibility. Rory played fine, but he was always at risk if players can go really, really low, and both Smith and Young managed it. On another day, Hovland goes low and wins by a landslide. In the end, Rory didn't play himself out of contention or anything but just didn't have enough putts in him to deal with players shooting mid-60s scores. It happens.