Of course some accounts get boosted, that was happening even before you had Storm League where it made it easier and cheaper to do. I once encountered a (now former) friend in my Hero League game, he was playing a hero I knew he couldn’t play and with skill he didn’t possess. It turned out he had paid a boosting service to push him from Plat to Master.
Even with this in mind, people do climb out of ranks on their own merits and it often takes much less than a season to do so. We were talking about this in another thread, you have to keep in mind HOTS seasons can now be six months long, it’s not like the golden days of 2-3 month seasons.
When I started my most recent season (in Plat) I hadn’t played for many months so in the first 10 or so games I was gaining or losing up to 600 points a match, this can move you around the ladder quickly. I’d guess that’s what happened to the person in question.