I never get any response from Okta support team for this topic. Okta documentation is not straightforward and you need to struggle a lot to collect information required specially for Xamarin(iOS,Android and UWP).
You can try following sample if that works for you. It is a basic OidcClient library which works for all 3 platforms in Xamarin.Forms . For me, after implementing it was not giving me valid Okta token(i.e signature was not being verified from Okta) because for that I need some customization with Okta Authorization end point and this library doesn’t provide any customization.(i.e I need to pass some extra parameter for Authorize endpoint as per my server configured for Okta)
From this article you can find this link for sample code.
But I end up having fully working Okta with manually calling all required Okta endpoint and I followed this documentation. Basic Concept
I went through the same example you listed and was trying to get OidcClient to work with Okta but not getting anywhere. Was thinking about having to do it manually like you suggested, but then that totally defeat the purpose of using the OidcClient, isn’t it? Really hope Okta team can come up with an example of using the OidcClient library, since it is working fine with the identityserver.io sites.