> All integrations were implemented in-house using the Texts Platform SDK. The SDK will be open sourced at a later date.

Seems like a big nope.

> Messages, contacts, auth credentials, account information never touch Texts servers.

> Your messages are sent directly to the messaging platforms.

> All end-to-end encryption is preserved when the platform supports it.

> Texts is a client and works like the official app.

Apparently all the code runs on the user's device.