This has a number of advantages over GPG. In general, it provides better default security without having to fumble to pick the right options with GPG.
For example:
In order to get sender authentication in GPG, you have to sign the message. GPG doesn't do this by default when encrypting a message, but you can choose to add a signature. But even if you do sign the message, GPG provides non-repudiation which means that you can no longer deny that you sent the message. Kestrel provides sender authentication while preserving deniability.
There are a number of other strong security guarantees that are provided by Kestrel's use of the noise protocol. There is more info on some of those properties here [1]
If your app is in a Docker container, you need to have some way to mount a persistent volume for the database. AWS Fargate and fly.io both offer storage volumes.
While Apple may not intentionally keep you from leaving their platform, a side effect of their tight integration is that it's harder to leave. Take the Apple Watch for example. Without an iPhone, the Apple Watch goes from a smartwatch to only being able to tell the time. Sure, you could own an Android phone and an Apple Watch, but you wouldn't get any of the benefits of the Apple Watch, which further ties customers to the iPhone.
[1]: https://techcrunch.com/2011/10/19/dropbox-minimal-viable-pro...