At the bottom of the article they mention:
What happened here? The tooltip was attached to the tooltip container and not to the body.
This invalidated a much smaller subtree, which was the tooltip container. The tooltip container
is not visible in the page, so modifying it doesn’t invalidate the complete page render tree.
If the tooltip container would have been visible in the page, then the complete render tree
would be invalidated but in this case only an independent subtree was invalidated.