Later on the article it reads

  So the day after achieving the highest score in WoW

So I suppose the initial sentence of "highest level" is not correct. I guess they are talking about "Server Rankings" - i.e. http://www.wowprogress.com/realms/rank/us

Which is very hard (apparently) to accomplish and I think they get reset now and then. There are other key achievements in WoW such as being the first guild to kill a boss etc.

So I think he was actually ranked #1 in the server, that's what "highest score" refers and that can take months.

I don't know much about WoW but I have a friend who still brags about his #1 in a WoW server 5 years ago :), so it's a big deal.