To be more specific about the error (yes the http is intentional, https immediately fails a ssl handshake, but http gives a possibly useful error):

> $ curl -i http://press.knowledge.dev

> HTTP/1.1 409 Conflict

[...cloudflare headers...]

> error code: 1001

google suggests that error 1001 is a cloudflare-side DNS configuration problem with the domain (which is CNAME'd to target.substack-custom-domains.com, which is cloudflare-hosted). CNAMEs to cloudflare seem to require special care.