It doesn't require your entry guard to be pwnt for an attacker to enumerate your entry guards, although it does require a pwnt entry guard for it to be used to link a client to their destination. As far as I am aware, the current research shows that a single packet is enough to link two parts of an observed stream together. There are millions of packets going across the Tor network at any given time but this does not prevent an attacker who can see only two packets from linking them together. For this reason I highly doubt that noise of other users exiting Tor will be enough to protect from this attack. Also it doesn't require compromised entry or middle node if it is used to compromise circuit isolation for exit traffic linking.