Why would this comment be flagged?
It's a reasonable statement, Twitter handles have replaced phone numbers in the age of online discourse. Telling someone to join a fringe social network is like saying people we can ban people we don't like from using the telephone because ham radio exists.
https://wireless2.fcc.gov/UlsApp/UlsSearch/searchLicense.jsp
One example of a community being labeled as right-wing extremists (or worse) is Gab.ai. Another Example is Zerohedge's comment section.
Was anyone doxxed for using these platforms? I don't know. I do know those platforms have been deplatformed.
There seems to be a concerted effort to label Facebook a place where extremists gather, lots of people virtue signaling about how they're not using Facebook because it's so toxic and full of Nazis. It's only a matter of time until it's a sin just for using Facebook.