HTTP auth doesn't make backend authentication code easier to write. In virtually all apps, it doesn't obviate the need to do session management, or solve the user demand for "remember me" cookies.
What it does do is shoehorn applications into a single inadequate login UX, one that (almost at the protocol level) is not amenable to basic security measures like gracefully logging out sessions.
Clearly there's quite a bit more wrong with HTTP auth than that (for instance, it does a terrible job of handling errors, it doesn't handle "forgot password", &c &c) but I'm not particularly interested in the counterfactual argument of what could have been had browser vendors only built a "good" version of it.
The fact remains that even the hypothetically "good" version of HTTP auth would at best be only marginally better than what we have now, and would leave all the hard problems this "definitive guide" is trying to address still on the table.