Feds already had the server, they didn't need to hack into it. The feds were trying to locate the owner of the server and collect the evidence.
That's the whole point of my first post. Read it again. Ulbricht's feeble authentication method was to only allow his VPN IP to connect to sshd. This was his decided method to keep out anybody trying to get into the open SSH port. When the feds found the server (through other means), and had the hosts in Iceland copy the server state they discovered this IP address in sshd_config, traced it and found him.
What I don't understand is why he didn't use TOR to connect to the VPN, instead of connecting to the VPN directly. (I think this is possible?) The government apparently had used pen registers to get routing information from the server in iceland, so I would assume they did the same with the VPN to get his actual IP address. However if TOR exit nodes were all that were connecting to the VPN with his credentials then it would be impossible to make the connection back to him (unless I am missing something).