Earlier quoted context omitted.
The Matrix identity server system is entirely optional to use, and basically a separate system from the rest of Matrix. The reason for that is that there's not really any way to implement that specific feature in a decentralized way, so they did the next best thing by making it a centralized server that you can choose to use (opt-in) in case you want people to be able to find your Matrix ID based on your phone number…
"Using the centralized identity server is entirely opt-in" So there are no channels on Matrix that require you to identify yourself before you can join?
Third Party ID's (email etc) are only used for opt-in contact discovery.