This is very incomplete, as you can easily see by switching to the aerial photo base map but keeping the tree overlay[0]. You'll see a ton of trees that are not marked. I'm guessing it's only city-owned trees, or something like that?
If you open the hamburger menu and go to "more maps", you can click an info button next to the tree marker, which shows a popup with the word "Stadtbäume" (city trees) which links to here: https://www.stadt-zuerich.ch/ted/de/index/gsz/natur-erleben/...
That page clarifies that these trees are indeed just the trees on public land, which are maintained by the city. Trees in private areas aren't included.
That page clarifies that these trees are indeed just the trees on public land, which are maintained by the city. Trees in private areas aren't included.