It's also not clear that the server must forward all the delivery receipts before it forwards the later replies.
Could it just eat them all?
EDIT: after a quick look at the spec¹, it seems that it supports out-of-order messages, meaning the server could selectively eat receipts.
[1] https://whispersystems.org/docs/specifications/doubleratchet...
So, given that, it would seem that a compromised server could pull a whole conversation (if people overlook the single tick mark), as claimed in the article?
I've not seen any specific claims about the mechanism for the delivery receipt - can you link me to this?
It's not even clear to me that the delivery receipt is signed.