The question is not whether they can lie, theoretically. Yes, I can be as cynical as anyone in the world, but that's no great intellectual accomplishment without insight. Substack has no practical motive to lie, because that would just alienate the Substack writers upon whom Substack depends essentially. If the writers leave the platform, then Substack is absolutely nothing, worthless.

Furthermore, "only signed-in users will be able to post notes, see the feed of Notes, and engage with notes." The inability to read feeds while not logged in makes Substack Notes much more hostile to casual browsers compared to Twitter. That's not a very eyeball friendly policy. I don't see how they can even be intending to replace Twitter that way. I'd personally want to see what Notes are like before signing up for Substack, but I can't do that.

Occam's razor suggests that the purpose of Substack Notes is straightforward: to create more engagement between Substack writers and Substack subscribers. It seems like a good idea for that purpose. But a Twitter replacement? If that's the intention, it's designed totally wrong.