I touched upon this in my response to your other thread, but to add an answer here, if you’re looking to secure your own API with Access tokens issued by Okta, it is recommended/required that you use a Custom Authorization Server.
Also, I do want to note/clarify here that the guide you linked to discusses how to use Access Tokens issued by the built-in Org Authorization Server is specifically intended to be used against Okta’s own API endpoints, which is what that guide is focused on. You would follow these instructions if you are looking to use OAuth authorization to access Okta’s endpoints instead of using an API token.