It's not a good idea to top autos because they have a short veg period and pinching them throws them off.Normally autos start showing sex at three weeks give or take a few days depending on strain.Your plant looks real healthy.Have you checked real close for preflower pistols?The only other thing is it could be a photo plant that the seed bank or breeder got mixed up.I would wait another five or six days and if you still don't see any pistols i would try putting it under 12'/12 lighting and see if that doe anything.Keep checking every day for pistols and let us no whats happening.