It will still generate traffic to Twitter in order to fetch the tweet. It’s an alternative frontend for privacy purposes, so Twitter doesn’t get your browser fingerprint, IP address, etc. It’s all proxied through xcancel/nitter.

And you don’t have to login.

However it generates only a fraction of the traffic that would otherwise reach twitter, and also makes it useless as it cannot be data harvested.

Ah, OK. That’s why they are pissed off.