Our app enables existing users of our telecoms platform to access their account. I'm confused as to whether or not this means I have to add Sign In with Apple to the app?
It doesn't make a lot of sense to me to do so (in that it doesn't exactly add value), but if there's some way our existing users can say "Let AppleID ***@yyy access my account" (which they would register on our platform) then I guess it's possible.
No it's not necessary in your situation.
According to the App Store Guidelines, "Sign in with Apple is not required if your app exclusively uses your company’s own account setup and sign-in systems."
https://developer.apple.com/app-store/review/guidelines/#sign-in-with-apple
However, if your app uses something like Facebook or Google authentication in addition to your own authentication system, then yes, you'll need to add Sign in with Apple.
