Although, it's worth remembering that the Chaplin sisters did actually both serve time for breach of the peace. (I think it's possible that what we now think of as stalking and harassment would then have fallen under the 'breach of the peace' umbrella; whether the charge was specifically in relation to the harassment charges, or their behaviour in court, is difficult to work out from the meagre reports available.)
A custodial sentence (albeit a short one) smacks of aggravating factors - I'm not totally convinced it's something that would be handed out in the first instance in a case like this unless other factors were at play, and it might indicate behaviour a little more serious and sustained than just being a bit disconcerting. (Their psychiatrist apparently believed that they suffered from erotomania, a delusional disorder connected to some of the more severe cases of what we now define as stalking - so not a negligible condition.)
I kind of doubt that they were actually dangerous, but it is quite easy to see how the kind of behaviour evident at around 18:14 in the full documentary could be psychologically very intimidating. In this case, the subject of their attention is their parents (who were allegedly reclusive themselves), but if they behaved in a similar way with a stranger, like the lorry driver who took them to court, then you can see how and why he might have felt at best somewhat put upon, at worst, quite distressed. I mean, I'm generally pretty robust - but having two identical twins stare at me over the garden hedge for hours on end might well bring on a dose of the heebie-jeebies.