open Google maps at Monument station, find a tree in the area. all the streets in that region of London (let's say 1 sq km) are quite narrow, I would guess there just is not enough space for street trees.

When I drop a pin to Monument station I see a sign, so I spin the view around. In canon street I see two trees (no leaves - winter). They're hard to see as they're behind a black cab.

Clicking once into Canon street towards those trees presents me with the trees. They're now in leaf and look like Sorbus intermedia "swedish whitebeam" and the key id is the margin on the leaf and the green fruits. Photo was taken July/August as prior to that they're in the flowering phase (beautiful to see btw).

When I spin the view down Canon street I see three mature trees in full leaf on pavements / sidewalks.

As I said in another reply, I'm an arborist and I'm hardwired to see trees and perhaps I subconsciously avoid areas that have none, so maybe that's bias on my part.

The article links to the Tree Equity Project (https://uk.treeequityscore.org/map) which has pretty detailed measures for London. Some very central areas do go as low as 2-3%, but they are probably the exception rather than the rule.