You can easily synchronize with sites 100s of km away (during my earliest testing, I regularly had sync between receivers in Cambridge and Eindhoven - close to 400km apart) because they can use ADS-B equipped aircraft flying at 30000ft+ to synchronize.
But mlat for an aircraft requires >= 4 receivers seeing that particular aircraft. For low aircraft, you need a high density of receivers to have enough receivers seeing them, because the radio horizon is much closer at lower altitudes.
Having lots of sync really only tells you something about your mlat coverage at 30000ft, not lower…
