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in your first example, how does Signal route messages in that model?

sender specifies recipient but the signal server cannot tell a group-chat message from a non-group chat message.

so in this world signal still knows I'm sending to N recipients? Or is the fanout happening in a different way?

I guess I'm wondering if Signal still basically has enough info to make good guesses at group existence.

Yeah I think you are correct that the protocol isn't safe from traffic analysis.