The Gunners are fifth and - like Chelsea - undefeated.
Di Matteo said: "I said at the start of the season Arsenal would be challenging for the league.
"I'm not sure anyone believed me, but that's the way I saw it and I haven't changed my mind. They have signed some very good, interesting players.
"But I don't think they are playing differently.
"The personnel has changed, but the philosophy hasn't changed.
"Arsene Wenger has done a fantastic job."