Hiya Colin!
Disclosure, I'm head of devrel at FusionAuth, as well as a co-worker of the author of the post.
We think there is value in separating identity in the way you describe for a number of reasons.
* While this tutorial is about one app, that isn't usually how things work beyond demo land. In most places you have more than one application authenticating against FusionAuth (a couple of custom apps, OSS software like forums, COTS like Zendesk). In this case, isolating identity to a single component allows you to manage it centrally and to, if needed, bolster security.
* If you use OAuth, you get access to a tremendous selection of libraries across different languages. Every language has an OAuth framework you can use.
* Developers can have portable skills. Sure, figuring out the Authorization Code grant can be a bit of a pain, but once you do it once, you can do it again and again.
* By standardizing on OAuth/OIDC, you can move between identity providers. Yes, there are differences at the edges, but you'll have more flexibility than if you code to say, Clerk. This may or may not be worthwhile.
If you want to see a take on how we think about authentication, the Modern Guide to OAuth is a great article to read: https://fusionauth.io/learn/expert-advice/oauth/modern-guide... particularly the "OAuth modes" section.