You have absolutely zero guarantees, with or without their policy on third party apps. You can not send sensitive information to someone else’s phone and tell yourself it couldn’t possibly have been intercepted, or that someone couldn’t get ahold of that phone, or that the person you’re sending it to won’t take a screenshot and save it to their cloud.
A lot of software nowadays is doing a real disservice to their users by continuing to lie to them like this by selling them the notion that they can control their information after it has been sent. It’s really making people forget basic information hygiene. No app can guarantee that message won’t be intercepted or mishandled. They can only give you tools to hopefully prevent that, but there are no guarantees.
Moreover, this policy does not exclude them from including third-party functionality and warning the user when they are communicating with somebody that isn’t using encryption.
Too many of these apps and services are getting away with the “security” excuse for what is effectively just creating a walled garden to lock users in. Ask yourself how you can get your own data out of these services when you decide to quit them, and it becomes more apparent what they’re doing.
If the first party provider told you this, you should always second guess them.
Moreover, providing an option that informed users can choose is doesn’t hurt security. This idea the user can’t be trusted to use the appropriate type of messaging if provided options needs to die.