How is Starship related to Brilliant Pebbles? It's Starlink that seems to prove that you can put a constellation of satellites in the tens of thousands affordably. That's the type of coverage that Brilliant Pebbles needs to succeed, right?
The reason Starship is the threat is that the kind of launch capability needed for a mars colonial fleet, and the turnaround times advertised by Musk, mean that such a missile shield could be manufactured in secret and then deployed in a few hours.