I think the 5 minutes of exercise per month of age relates to enforced on lead walking rather than just running about playing.

I agree. My puppy spends much of the day bouncing off the other dogs and playing rough games. Going out to be socialised or for a walk with the other dogs where she potters about sniffing must give her joints a rest from her boisterous puppy games?

You see you are taking it all completely the wrong way!!!
No one has said your puppy shouldn't be allowed to run free or play, as you have said they rest when they have had enough, and that is the key. Onlead walking is Forced exercise if the puppy gets tired, they aren't allowed to flop until they want to move again, that is the difference. So taking ONLEAD WALKING at 5 mins per month of age ensures that this doesn't happen.

when i talk about free running i am not refering to the garden but the fields and woods where mine go from 8 weeks - that is top of going to the clubs, visits into town, bus/train rides and training, visiting peoples houses where there are other vacinated mentaly stable adult dogs of other breeds - you name it mine do it apart from jumps