I'm interested in a explanation for that too. Signal recently introduced the same sender key method for groups [1] and, correct me if I'm wrong, this is first formal mention of the "Selective Forwarding" method we have.

I get that Signal's open-source implementation doesn't store this type of metadata, but is very misleading to see the founder of Signal tweeting that "doesn't have access to the data" [2] while it's not that much better than Whatsapp in the technical side.

[1] https://community.signalusers.org/t/sender-key-feature/33599...

[2] https://twitter.com/moxie/status/1457005910136549379