Scroll down this list of search results (on the search term substack.com) and you'll see a history of submissions by many different users over the last 10 months:

https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...

I don't think this looks suspicious in any way.

Maybe the people who have become disillusioned with medium.com have found an alternative blogging platform?