The reason behind is that you need 5-7 turns to get a worker and some more turns to get a total of hammers superior to 10(reaching 3-4 citizen). At 2 citizen you can build a settler between 7 and 9 hpt and ''lose'' only approx. 2 turns of build. But since that you build a settler 10ish turns faster, the 2nd city will have enough time to add lost hammers and overall growth overcome the ''losing'' deal.
When you do that you can expect settling a 2nd city as fast as the turn 15-18 which is faster than getting a free settler from Liberty. The mechanic favourise an early 2nd city since that you can stay happy enough before getting your first lux if you find at least 1 NW or simply settling on a lux tile. This also let you block a chunk of land if you spawn near another capital.
Of course, you might want to explore aggressively(start with 2 scouts) and try to get lucky with a worker bought if you reach 220 gold. You can also get a free citizen while building a settler too
But doing that way means that you have to let down Liberty in most cases. If you go Liberty i suggest to build at least a monument and start settlers only when you reach the free settler policy to get maximum benefits from that tree.