Do you see any errors returned when you try to exchange the sessionToken for an authorizationCode, for example an error that says “login_required”? If a 200 response is being returned, you’ll need to check the actual page to see if an error is returned or you could check your Okta System Log for any failures for the OIDC app in question.
Yes, that article should help you ensure the user will only be challenged for their Password. You may also need to double check the Global Session Policy in your org to double check that it also will allow a user in with only a single factor/just their password, as in this article: How to Prompt for Password Only when Logging In to Okta (OIE)