Correct, they'll only be broadcasting in certain areas where the partners are permitted to operate, using their spectrum. At the moment there are few regulations on mobile providers in most places, as stated in the article. In the past that wasn't a big deal because most radio observatories are remote and many are typically situated in valleys or bowls so the terrain helps block the interference, but that may not be enough anymore.

The clocks may be stable but the Earth is changing all the time so it still has to be updated frequently, not just time offsets but position offsets to account for tectonic activity, changes in the Earth's rotation and precession, etc. It is all very small but over time the errors grow.

Directional antennas still produce sidelobes. They aren’t laser precise. Keep in mind radio telescopes are very sensitive, a satellite like BlueWalker 3 would be far stronger than natural radio sources in the sky even if it isn’t directly aimed at it… and the goal is to have enough of them to maintain coverage of an area at all times. The point is to at least not directly aim the beam at a radio telescope, and provide a little extra buffer room to account for the sidelobes.

It would help a lot, and that’s exactly what radio astronomers are proposing. The FCC required Starlink to create those gaps, and they aren’t perfect. Enforcement is lax and the protected areas aren’t big enough so radio observations are still impacted by Starlink, but it’s still a big improvement. Basically radio astronomers are asking the FCC to treat 5g cell frequencies with the same care and consideration that other frequencies already get. The Government Accountability Office (GAO) wrote a recent report that could be interpreted as saying the FCC needs to be looking into this and other concerns.

It’s important to note that the FCC has not granted permission for AST SpaceMobile’s plans yet, they only have an experimental license for one single satellite. So there is time to act and come to an agreement. But compared to SpaceX, Blue Origin, and others, AST Spacemobile has not been very cooperative about working with astronomers. They ignored dozens of invitations to meet and discuss the problem, various conferences on the topic, etc, and are only just now in the early stages of discussing it - but have made no public comments or promises.

ELI5 version: watch this 2min video by NASA on what geodesy is

https://space-geodesy.nasa.gov/multimedia/video/vlbi_quasars/vlbi_quasars.ogv

It won't affect communications with GPS satellites. But those satellites are only useful if they have a reference point on the earth to calibrate to. Those reference points are generated by radio observatories that observe distant quasars, similar to how sailors used to observe stars, to determining the Earth's precise position in space. But those radio observatories primarily use frequencies that are in the same range as cell towers, so they're in remote areas. AST SpaceMobile's plan is to basically flood the earth with these signals so these remote areas are no longer "quiet", rendering a large portion of the radio telescope's observations useless (exactly how much is TBD).

GPS satellites tell you where you are on the Earth. But how does the GPS satellite know where it is? Geodetic observatories tell the GPS satellites where they are relative to the Earth. Without that, it's all going to be inaccurate.

I think if more people realized what radio observatories did they would care a bit more. Radio observatories do more than just look at things in deep space, they also are crucial for geodesy- the science of the Earth's shape and orientation in space. They do critical work in keeping GPS accurate, measuring the drift of continental plates, measuring sea level changes, calibrating navigation systems, calibrating spacecraft, etc., and all of these functions require them to be based on earth in radio-quiet environments. This work cannot be done from space.

So, in a very real sense, satellites like the proposed BlueBird constellation will interfere with GPS accuracy for the world. NASA is investing millions into a brand new set of geodetic observatories around the world that could be sensitive and accurate enough to even help forecast earthquakes and tsunamis, provided they can do their work without excessive interference.