Whilst I absolutely agree it's correct to be skeptical about it, the 'sealed sender' process means they don't actually know which account sent the message, just which account it should be delivered to. Your client doesn't even authenticate to send the message.
Now, I'm just going on what they've published on the system, so either I could be completely wrong, or they could be being misleading, but it does look like they've tried to address the very issue you've been pointing out. Obviously it'd be better if they didn't have your phone number at all, but this does seem to decouple it in a way that means they can't build a connection graph.
'Sealed sender' seems to avoid this by not actually requiring the client to authenticate to the server at all, and relying on the recipient to validate that it's signed by the sender they expect from the encrypted data in the envelope. As I mentioned in another reply, I’m just going on what they’ve published on the system, so either I could be completely wrong, or they could be being misleading, but it does look like they’ve tried to address this issue.