The "fact that a user is using Tor" is not discussed in the post. There is zero connection between how Tor nodes generate their TLS certificates and whether or

The "fact that a user is using Tor" is not discussed in the post. There is zero connection between how Tor nodes generate their TLS certificates and whether or not you can detect that a user is using Tor. All you can do with this information (which is not a secret but a well-discussed tradeoff with no better option) is to identify Tor relays, which are already public.