As none of our Lawyers has posted (as yet), I’ll offer a bush-lawyer’s comment, and that’s that you’re spot-on Erin!
The Arborist should have availed themselves of the Dial before you Dig service as opposed to their meaningless disclaimer, but that said as Telsta has used authorised contractors (and their sub-contractors) for so long, their Cable Plans are known to be very much advisory.
Telstra probably does have the right to lodge an ambit claim against the Arborist, but it certainly has none against your Owners Corporation.
I’d strongly recommend that your Owners Corporations keeps up the dialogue with Telstra, because as your Strata Plan (and its residents) are the recipients of the services provided by their cable, any debt-recovery “incentives” that they may employ could affect those residents as the innocent third parties.