Why is Telegram’s server-side code closed source?

Why is Telegram’s server-side code closed source?

Why is Telegram's server-side code closed source, but the client code is open source? Does having closed source servers improve its security?

Telegram FAQ page:

Q: Can I get Telegram's server-side code?

[…]

By contrast [to the client apps], publishing the server code doesn’t provide security guarantees neither for Secret Chats nor for Cloud Chats. This is because – unlike with the client-side code – there’s no way to verify that the same code is run on the servers.

I don't understand this explanation. Verifiable builds could be used with server code, too.