He's not "policing", rather giving good advice. Substack is only one provider for newsletter producers. Calling your newsletter "a Substack" is akin to calling your blog "a Wordpress", and a number of successful writers there have transitioned to the competition and done better.

If you call what you have a Substack and then you move it, what do you have, and will people think you disappeared when you just are self-hosting or moving to Ghost, Buttondown, Beehiiv or someone else?