FM1 says you are still "on duty" for that 15 minutes, as a reserve pilot on duty and not mid sequence you are subject to reassignment.
That note just says that the 15 minutes cannot be removed from your duty day for the purposes of determining rest. You are still "on duty" either way, it just protects you from having CS squeeze in an extra 15 mins to your duty day by getting rid of the 15 min debrief.
The contract allows you to self release with HI3 & HI6 immediately after block in. The 15 minute post flight debrief is still considered for the purposes of determining rest.
By all means, if you think this is all silly crew room business then don't check your HI3&6 and don't pick up your phone when CS calls 10 mins after block in and leaves you a voicemail with a valid flight assignment. Let us know how the grievance goes.
There's no reason I would know any better than the next guy - but to me this "myth" is spelled out pretty clearly.