I wouldn't expect they are rewriting their user agents to announce their company. You'd get a regular user agent if they were using a regular browser. It feels

I wouldn't expect they are rewriting their user agents to announce their company. You'd get a regular user agent if they were using a regular browser. It feels more likely that they are gathering stats on tech writers and weblogs to see who they might want to invite to Substack one day. They could hide this with a fake user agent, but have chosen not to.