I know this is kind of a joke comment but the community is tiny:

https://write.narwhal.space/u

23 users

7 users that are semi-active (more than 5 posts)

None of which seem 'professional writers', but it's hard to guess from an empty bio and a random username.

I would guess more of the latter but at least some of the former. I would be very surprised if Cory Doctorow and Charles Stross didn't have HN accounts

Ouch.