Context
Why everyone remembers it as a Jordan line
The commercials were sold as Michael Jordan commercials, and Jordan is the reason anyone watched them. But the script hands him almost nothing: Mars Blackmon does all the talking, cycling through guesses about what makes Jordan the best player in the universe, and Jordan answers each one the same flat way. The memorable sentence is the one Blackmon arrives at on his own, against Jordan’s explicit denials — which is the joke. Jordan spends the spot saying it is not the shoes.
That inversion is also why the line worked as advertising. A pitch delivered by the athlete would have been a claim Nike had to stand behind; delivered by an obsessive fan whom the athlete keeps correcting, it becomes a wink. The credit has drifted to Jordan for the ordinary reason that celebrity absorbs everything shot near it.
Jordan is the one you remember. In the ad, he is the one insisting it is not the shoes.
Who actually wrote it
Authorship is genuinely split. Lee performed the line and directed the spots as Mars Blackmon, a character he had already written for himself. The campaign itself came out of Nike’s agency Wieden+Kennedy: Jim Riswold’s Wikipedia entry ↗ states he “created advertising campaigns such as the Mars Blackmon (Spike Lee), Michael Jordan commercials for Nike.”
No published shooting script for the spots has been located, and the sources that quote the ad quote it from the broadcast rather than from a document. That is a real limit on how precisely the wording can be pinned — but it does not touch the attribution question, because every source that names a speaker names Blackmon.