What this article misses is a lot of the complex formatting issues also with text rendering. For example even implementing tabs can be complex (remember that you have different tab types and conflicting constraints like keeping something decimal or left aligned but now having enough space to do that with out overlapping characters.) In languages like German if you have a computed or soft, hyphens the spelling of a word can change. Good paragraph breaking, widow orphan control, page breaking, computed header/footers that can change heights… are also complex issues that have to be dealt with.
Back when I worked on this stuff we also had much slower computers that made things even more difficult so that you could type anywhere in the paragraph and still have responsive and correct output (you can’t delay formatting if the chargers change on context although some formatting can be delayed.)