In that situation I (personally) would've moved him.
You can't put 'dibs' on public areas. I know you really wanted him to have a sleep, but why could he have no sleept somewhere else at the park?
I personally never look in other peoples prams & if I saw a mum & 2 bubs playing on the equipment, & a double pram parked away, I would assume the pram belonged to the two playing kids.
They probably sat down & figured if you thought it would be too noisy you'd move him.
Do you live in the city/built up area? I do, and sometimes I feel like strangers are living on top of me & disrespecting my personal space. But really they are just trying to live their lives.