I used to play a really bad deity start (one that at T100 I had single digit science and culture, and my 2nd city was about to flip due to loyalty) to see if I can actually win. One AI did got ahead and even near T500 (it was a game with x2 mod so things are slower) he has more science than me. Eventually he declared war and nuked me, so that was an eye opener for me. But then he nuked the same city again and again . If AI knows how to 1) take city immediately after nuking, or 2) nuke multiple targets, it would be a lot more threatening.