Hacker News Reader

If you check their costs, SMS (used for registration) is the most expensive part of their operation.

There was no "registration" and TextSecure never sent you messages. It was strictly peer-to-peer.

You're mistaken. I still have my textsecure account registration verification text message (because I'm a data hoarder) from March 15, 2015

2015 was after the internet-based-messaging transition but before the rename. Source: https://en.wikipedia.org/wiki/TextSecure

You misunderstand how textsecure worked.