If Marseille can maintain this good form in the many match to come including when they away to PSG, then I can say they deserve to be strong contenders for the title. But it is something that is difficult even though until week 7 they are unbeaten.
Marseille will face PSG in mid-October. This is PSG's home match. If Marseille manages to beat PSG in this match then I would say Marseille is a worthy contender for this season's title. If PSG wins against Marseille then we will see PSG dominate this season as in previous seasons. And PSG will win the title at the end of the season. Personally, I think Marseille will lose their consistency very quickly and start going down the points table.
It's still a long way from that I think because now is not the time to think about that for Marseille because the match against Rennes will be tough, not to mention they still have to rotate players by preparing for the 3rd meeting against Sporting CP in the Champions League so instead of thinking too much about the match against PSG then it would be better for them to think about doing well in their next match