I'm pretty certain Tango mentions it in the Iron Titan tutorial you linked. In the end, it all comes down to village mechanics, i.e. what the game considers to be a village.
Minecraft normally groups all eligible doors (i.e. one with more sky access on one side compared to the other) within 64 blocks of each other into one village. Big iron farms make use of special mechanics to stack villages on top of each other. The Iron Titan uses 32 or 64 villages right next to each other to massively increase the number of golem spawns compared to a regular farm with a single village. The basic idea is to place one door, creating a village. Next, you create another village by placing a door 65 blocks away. By placing and destroying doors in a very deliberate way, the second village can be moved until it's next to the first one. This is what the large "arms" of the Iron Titan are for.
The problem with this technique is that Minecraft recalculates villages when the chunks load. The game will see the huge number of doors in one space and make it into a single village. The only way to prevent this is to prevent the chunks the farm is in from unloading. The spawn-chunks, i.e. the 13×13 chunk area around the world's spawnpoint, are always loaded as long as any player is in the overworld, making them ideal.