I think the "not allowed to exercise" is meaning, no strenuous, repetitive exercise, like a 5 mile jog on pavement. Sprinting around unrestrained on soft dirt and grass sounds like great fun to me, not "exercise".
I just don't understand how you can expect these big, muscular dogs to grow correct muscle density if they're not allowed to use them... but... I'm not a vet or even a long-time mastiff owner... I've had big dogs for the past 15 years, but Denna is our first full mastiff (EM).
I let Denna lead our walks from the second week she came home... at that point I could still carry her so the old adage of "don't walk a mastiff further than you can carry them home" was in full effect.
But, by the time she was 6 months old, she was very happy to take a one mile walk on cool days. She often wanted to pull me along to go faster, so I've gotten in better shape as we've both gained in speed and distance as she's aged.
She's 15 months now, and is very happy to go for a 3 mile walk... with some jogging and off-leash sprints mixed in. After mile 3, we're both ready for a lounge chair and a cold pint...