It wasn't plastic! It was landscape fabric (what we call it here in the US - whoever it was that put it on for Michael said it was called "bash" in France - at least I think that's what he called it, it was hard to hear/understand him). I also don't believe the garden was fully planted, for whatever reason. Towards the end of the video, a lot of the pots were visible, showing above-ground. I think the landscape fabric prevented them from digging the hole - you have the cut an "X" through the fabric for each plant and then dig. Maybe Michael was ready to film and just wanted to get on with that and will go back and truly plant! Let's hope so, as it was quite pretty!
In my experience, I would never lay a plant suppressing membrane in a garden that will ordinarily, organically mature over time - absolutely no need! Especially as his "piano" area didn't appear to have weeds, e.g. rather good soil imo, and with the additional organic matter from the forest, even more so. Mick knows his stuff for sure along with Sherrie.
Plus, self seeding is great for creating more plants too for free. And, if you don't want the self seeding element, then you just weed out the seedlings that you don't want in any particular position. Simples!
Have had to do the same in my garden in connection with Forget me Nots - beautiful, but can become such a nuisance for other plants to receive enough sunlight etc in order to grow at this time of year.
Michael is correct, planting close together eliminates an awful lot of weeding too. Has always been my philosophy and works well. I love formal lines BUT, 100%, informal planting that flows over the same formal lines. They chose some great plants but, some of them were mis-placed in connection with height etc. No real problem, because once everything grows and they recognise which are being overshadowed etc, especially the hostas, just move them around at the end of the day.
Gertrude Jerkyll was the master of the English Cottage Garden style - Beatrix Potter just loved gardening.
Hey Michael, if you are reading here, bulbs would be an additional bonus too, when appropriate Daffs, tulips and Nerine bowdenii are really easy and add additional interest too throughout the year.