Where's the problem?
"At issue is whether NBN Co should backdate its payments to 2011, when the deal was announced, or 2012, when Telstra shareholders ratified the agreement."
An agreement cannot take effect until *both* sides of the deal agree to it. Since Telstra ratified (i.e. accepted) the agreement in 2012, then that's the date the agreement came into force.
Or am I missing something other than Telstra's usual all-we-can-get attitude?