Very Long Baseline radio astronomy has been around for decades. One of its early accomplishments was the realtime observation of plate tectonics motion between telescopes. These days that is observed routinely with high resolution GPS.
One EHT speaker I asked said an important advance was scaling up the VBLI procedure a thousand times from hundreds of megahertz to hundreds of gigahertz to achieve resolutions needed to image the three largest super massive black holes. This involves huge improvements across the entire system from the antenna receiver, clock resolution and petabyte data handling.
The EHT can resolve the two super massive black holes which are the largest on the sky. The Milky Way's SMBH is a relatively small one, but it's close enough to make up for that.
One EHT speaker I asked said an important advance was scaling up the VBLI procedure a thousand times from hundreds of megahertz to hundreds of gigahertz to achieve resolutions needed to image the three largest super massive black holes. This involves huge improvements across the entire system from the antenna receiver, clock resolution and petabyte data handling.