I would say instead that they just chose the highest security setting, which happened to require your phone number, a security setting defined by Discord, and optionally chosen by admins as the most secure option.
However, if you wish to disagree that’s fine, but you could absolutely say that the server admins have successfully handled their server security by using the highest security setting available. Just because you do not agree with using your phone number with your Discord account does not mean that they should or will make the change and it does not make them liars to describe that Discord is the official party middle man requesting your phone number at their behest to verify you.
It unfortunately seems that such a community may not be for you.
As stated before, they choose the highest security possible, and Discord as a service defines that as tying a phone to the account.
You can be upset all you want, but it is by definition, Discord requesting your phone number as a neutral party in the interests of both you and the server in order to be what Discord has defined as the most secure method of positive identification.
You don’t have to like it, and there’s nothing saying that the admins like forcing phone numbers to accounts to be present in their server, they’ve only deemed it necessary to ward off problematic accounts. The community they run is theirs and that decision lies in their hands.