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.
Here’s a link for my city. The city is number 1 for trees in the USA, I think Chicago has the least. At first glance it’s not as detailed and a little dated but there are more trees now than ever.
[0] https://www.maps.stadt-zuerich.ch/zueriplan3/Stadtplan.aspx?...