I gather the 10 minutes are, more or less, an industry standard.
nope, its not industry standard, we have planes with 5 and 10 minutes takeoff thrust.
prescribes 10 minutes of max continuous thrust to be available.
Whats your concept of "continuous"?
Look at the definition of stage 2, its based on takeoff power, therefore if the takeoff limit is 5 minutes or 10 minutes will depend on the AFM limitations for takeoff thrust.