I'm so skeptical of these kinds of stories.
I live in a deprived area. Our kids are a little behind compared to middle class kids. But not that far behind!
I'm just not seeing these 4 and 5 years olds who aren't potty trained or who can't sit on a carpet.
As for Americanisms: language changes, so what?
Long ago my Dad used to run a playgroup and, for a time was treasurer of the national pre school play groups association.
He came home once with a story about a playgroup on the ground floor of a tower block. All the playgroup kids lived in the block, or surrounding blocks.
One day it rained and all the kids ran to the window to watch.
They had seen rain pass by their windows. But, poor things, they'd never seen it hit the ground.
This was bollocks of course. But the story circulated because tower blocks were relatively new and people had anxieties around them.
I haven't heard anything recently about kids that don't understand rain. Because that anxiety has passed.
All this "screen time" stuff will be the same.