It's not mentioned in the article but Vim (and NeoVim) keep undo/redo history in a tree rather than a simple linear timeline[1].
The practical usage is it's aways possible to get back to a previous state you were in, which is pretty neat.
e.g. You can undo 5 changes, try something else and decide that you prefered the text before you started undoing things. In most programs with a linear undo history you've wiped out your previous changes but not in Vim.
You can hop about the branches of the undo tree using the g+ and g- commands but it's much easier to add an undo tree visualiser plugin such as the venerable old Gundo[2].