It did not unlock the first time for me following the guide and using the Bruins. It was my first GM Connected league and I had no others going at the time.
I got the achievement to pop on my second attempt. I moved my user profile to USB before starting the league. I was still in another league and didn't leave at any point, but it didn't seem to affect the achievement popping. It was a little late when I set up the options, so I ended up with it still having 7 game playoff series. To make it simple to win a game if I needed to, I had it on Rookie with 5 min periods. I advanced through the season and made the playoffs as the Canucks with no problems. The seven game series turned out alright cause I could sim one game at a time and only played if I was facing elimination. Even when facing elimination, I would start as a sim and then intervene in the last period to try and keep the time spent playing short. In the Finals, I simmed game to game until I could clinch. I played the final game and the achievement popped when I went back to the GM Connected menu.