Am I missing something?
Both my phone and my Sony TV (with Android TV and Chromecast built-in) require me to be signed into their respective Netflix apps. That is two logins.
Since my TV has Chromecast built-in, shouldn't I only need to login to my phone's app? If I had an older TV with a separate Chromecast plugged in, I would only need the one login.
Correct. You only need to be logged into your phone to cast to the TV. I found though, especially on my Sony TV, that it was far more reliable to use the app. I don't know why it is making you sign into both.