Hi there, thank you for the notice.
The process and UI really has not changed that much, but the link is now appstoreconnect.apple.com/apps. Meaning, you will find a similar page for each screenshot shown in the guide, but on this URL, instead of iTunesConnect.
TestFlight was and still is optional for testing - the easiest way is to deploy to your connected device directly, via XCode, so you are able to watch the compiler and build logs during deployment. There is no need to upload a build to AppStore Connect in this case (which makes it faster). Are you sure XCode does not show a single message regarding Unity IAP? If something is misconfigured, that would be the first step to look at.
I will look into updating the guide screenshots on Monday. Thanks!