They'll be using sidelobes. This is what's done at geo altitude which is also above the GPS orbits. At geo the commercial satellite I worked with had position

They'll be using sidelobes. This is what's done at geo altitude which is also above the GPS orbits. At geo the commercial satellite I worked with had position accuracy within about ten meters, and we always had access to 6-8 GPS satellites at a time. Obviously at the moon the signal is much fainter but my understanding is that it's essentially the same just harder to detect