Yes, it is. Nitter is just a front end for Twitter, doesn't host anything, doesn't have anything to do with what's posted on Twitter, it just shows it.
Twitter would be the party mailing the letter to Nitter, and Nitter is mailing it to the end user. Nitter is responsible for mailing it to the end user. It doesn't matter that they originally got it from Twitter or aliens from Uranus. The mail company is Firefox browser.
