There is a mistake in your logic. The amount of time you are in defense doesn't depend on the pace of the game but on rebounding and interceptions/turnovers.
If you face a slow attack, you will be defending more time by possesion but during less possessions (say 20 seconds x 60 possessions) and against a fast attack, you will defend less time but during more possessions (say 10 seconds x 120 possessions). There is no difference.
No he is right, because you made an mistake in your logic the opponent have an attack betwen those too attacks. So let this take 15s then your example looks like:
Slow attack: 20s* 60 + 15s* 60 = 2100s
Fast Attack: 10s* 120 + 15s * 120 = 3000s
So you need to have nearly 50% more playing time for the same amount of possesion, with relativ short attacks against fcp and long attacks against your defence this will be even worse. BUT i am not sure, if the engine calculates the expense like that, a easier implementation which also makes sense would calculate a stamina loss for every position which would favour the fast attacks.(the halfcourtgame is normaly less exhausting then the transition game or in this case the real pressing game)