It could be a fork, but forks dont last past two or three blocks. Unless you're the target of a massive double spend its not a big threat. So simply waiting for more blocks is good enough.
So if you imagine blockstream as an attacker, they could broadcast their version of the chain which say doesn't include payments they made on the main chain so that they could "double spend" them in places that rely solely on the satellite for truth.
Or am I missing something?
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edit: I guess there's no way for blockstream to create blocks at the appropriate difficulty level fast enough, so it would be pretty obvious if they were trying to create their own fork. So I guess this does work..
Edit: responding to your edit. Yes a good point!