I disagree. They would not need to access the full contents of outgoing SMS to perform this duty. For example they could see the auth codes masked.
Are you proposing they add an API where senders can annotate part of their message as private? (Not a bad idea...)
Do we know that Signal was using the Twilio Auth product and not something custom on top of Twilio?
Furthermore, it still doesn’t matter whether Signal was using their authy service or not. There should be very tight data controls at Twilio where few employees would ever be able to retrieve clear text messages being sent to end users.
This incident is not getting nearly the attention it should imho.