At some point during the win-streak, aren't you really just competing against the seed generator for a more favorable bunch of seeds that let you have the longest win-streak?
An achievement, yes, in the same sense that a speedrun is an achievement, and improving it is also technically an achievement.
But in my mind, the title of a competitive game are reserved strictly for games where the gameplay itself is performed against another player or a team.
Not to belittle his achievement, I'm sure it's very meaningful (to him).