I think in the situation the OP describes, the other mums knew she was on her way and that's why they didn't get involved.
I think if she'd nipped to the loo and he'd fallen then somone would've picked him up until she got back.
This is going to sound patronising and I'm trying to word it so it doesn't but the reactions of a PFB mum are so different to a mum of 2 or more.
When I just had DS1 I would've thought 'you heartless cows!' Picked him up, cwtched him, kissed him, checked him all over
Now with DD, no 3, I stand her up, check her all over, usually just a bump and say 'oh look dd, you're fine, now give me a kiss and off you go to play big brave girl'
She has been toughened up by having 2 big brothers.
And I can tell from how she cries if she is hurt or just shocked from falling/bump etc, whereas with my own PFB every cry was heartbreaking for me.
I would always help if a child was lost or on it's own. I think the reaction of parents is to do with their own panic, a bit like when DS1 has gone out to play and wandered off. When I have found him, after running all the horrible possibilities through my head, I have an initial Thank God you're safe moment with him, and then I just want to give him a good bollocking!