We have the same rules but I also understand that my children are NT and as such, I don’t get to make judgements on other parents or their children whose circumstances I have zero information about other than one of their children has meltdowns.
My eldest has SEN and was not enabled to scream in the garden any more than DTwins are now. Calling parents out for their shit parenting, is not judging. I get really annoyed with this trying to deflect and excuse shit parenting with words like SEN or disabled, that people are supposed to be intimidated by, as any further comment they can be accused of being "disablist" or "just don't understand children with SEN". And they're neither. I've got 13yrs experience of SEN, my own child, and support groups. And I'll tell you right now, there's a hell of a lot of shit parents who do nothing to help their child, then dress it up as "how dare you comment on his behaviour, it's alllllll his SEN". There are also a lot of parents who don't hide behind SEN as a get out of jail free card, and the difference in how their children are behaved is notable. And yes I'm well aware every child is different. I've seen enough over a decade to know it's frequently not down to the child. Being a SEN parent is hard. Bloody hard at times. That's half the reason why some are too lazy to do it.
Firstly, this child doesn't sound like he has SEN, it's only being thrown around because people have just jumped on the word meltdown and made their own association. Try swapping OPs use of meltdown, with tantrum, then perhaps this can be discussed properly, as a screaming brat that isn't parented.
What exactly do you think parents of SEN children should do, lock their children indoors?
And this is exactly what I'm talking about. 
Unless I let my SEN child do what he likes, scream, shout, disrupt, then the only other option is to "lock him indoors". Please.
The other option is to parent my child. To take him inside and let him return when he has calmed down. To go outside with him, and address what is causing the screaming. To take him to the park, where the noise level is acceptably much higher. There are many things I can do rather than churn out the lazy excuse of "ohhhhhhhhhh, I should lock my SEN child indoors, should I?" as if that's any kind of answer, or remotely what is being suggested.