Author here. This site lets you put in a username and get the users with the most similar writing style to that user. It confirmed several users who I suspected were alts and after informally asking around has identified abandoned accounts of people I know from many years ago. I made this site mostly to show how easy this is and how it can erode online privacy. If some guy with a little bit of Python, and $8 to rent a decent dedicated server for a day can make this, imagine what a company with millions of dollars and a couple dozen PhD linguists could do.
Here's Paul Graham:
https://stylometry.net/user?username=pg
Here are some frequent HN commenters: (EDIT: Removed due to privacy concerns)