You need messages from multiple receivers for MLAT, I think they use messages with known plane positions to calibrate the MLAT solver since the receivers doesn't have a high quality common time source.
Plus proximity doesn't mean you can see airplanes that are behind buildings, at low altitude, etc.
What is MLAT in this case? The planes transmit their location in the ADS-B message. They are filtering a lot of redundant message receptions from multiple receivers. I would assume the messages also include altitude and heading information, so locations can be predicted between message receptions.
MLAT is multilateration, where they use the difference in time of arrival to multiple receivers with known locations to estimate the positions of planes that doesn't transmit their location using ADS-B but only use older Mode S. This is mostly small or military planes.
Plus proximity doesn't mean you can see airplanes that are behind buildings, at low altitude, etc.