I added 2 more AIs in order to reach the maximum number allowed, with the plan of either Axe-rushing or Chariot-rushing. Since I needed Animal Husbandry early on with the map that I played out, it ended up being Chariots, although I lost quite a few Chariots against AI Archers and had to forgo early Wonders for a while just to get enough Chariots.
I made use of the cheap Workers and cheaper Settlers to spam a lot of them--the AIs were not really good candidates for Worker spam, so I took the AIs' Cities and self-built my own Workers.
I used Carracks as a means of delaying Astronomy for almost as long as possible, while still grabbing a lot of the good spots on the "New World" continent. Doing so meant that I greatly extended the life of The Colossus, at the cost of not sharing Resources with the "New World." Since I was able to hook up a couple of Happiness Resources relatively early on in the "New World," this delay in sharing Resources across of the continents didn't really hurt me that much.
Corporations were used on the path to victory, but building and spreading the Executives take a lot of real-life time, so I missed submitting my game for the corresponding Gauntlet by quite a number of days.
As opposed to denying the surviving AIs the chance to build Cities in the "New World," I used our cheap Settlers to build Cities for those AIs, essentially using our Imperialistic Trait as a way to really "walk the line" in terms of the Domination Land Limit by having AI Cities produce competing Culture.
There was a point where I was set up to create a few island Colonies, but I couldn't figure out how to make that option work. I did some testing and then I found out that the "No Vassals" option, which I'd left checkmarked from a previous game, meant that I couldn't have Colonies. I'm still not sure which option is better--having the extra Happiness from a couple of Colonies would have helped a lot, but then having to worry about a Vassal pushing us over the Domination Land Limit would mean a lot more planning would be involved--probably building Privateers just to sabotage your own Vassals' Galleons is a way to control AI expansion, but doing so also requires a lot of monitoring and can be subject to failure when your Privateer inevitably loses one of many naval battles.
I avoided him getting astronomy or flight, but none the less he managed to settle cities on the other continent, which greatly puzzled me and which made keeping under the domination limit quite a challenge.
It is possible for an AI to build Transports (via the Combustion tech) and thus send Settlers overseas without having access to Astronomy or Flight. Perhaps that's the situation that you experienced?
Certainly, with Colonies, such unexpected happenings are even more likely, since a Colony will get all of your techs at the time of founding the Colony, and even if your Colony doesn't settle overseas, it may trade the techs that enable overseas settling to your Capitulated Vassals or Peace Vassals.