A reasonable criterion for inclusion should probably be whether the service makes the protocol accessible for arbitrary consumption, whether openly or by authenticated subscription.
The phrase "supports TZDIST" can of course mean several different things, but while there are benefits to using the protocol internally to a product, I don't think this fulfills the expectation that it would tend to evoke here. From the perspective of someone reading tz-link.html, it's much more important to answer "where are some public tzdist servers I can point my project to?" rather than "what's something that happens to speak tzdist to its own clients?" (The latter, frankly, is of little relevance to consumers of the tz project data.)