If you just took a standard antenna that approximately speaking broadcast evenly in every direction, you couldn't (or at least you would have to bother triangulating the signal). If you did that though you wouldn't be able to offer high speed internet because you would be incredibly bandwidth constrained.
Instead, what these satellites are doing is using something called a phased array antenna, that let's them narrowly select what area they are broadcasting to and receiving from at any point in time with some fancy electronics controlling an array of many little antennas that constructively/destructively interfere. As a result, they have to know where the base stations are reasonably precisely.
I'm not actually sure how they discover where base stations are, my guess would be GPS on the base stations and omni-directional signalling to find them, in which case they know to within meters where a base station is. If that guess is wrong, you might be able to be a small amount over a border and have SpaceX not know it, but not substantially.
