Signal is closed source, considering their server code hasn't been updated in 9 months yet changes have been made on production since then. [1] This is a really dishonest argument.
He is saying telegram clients are open-source AND verifiable so you can prove the app store is giving the pushed source.
And there is no point of open-sourcing the server because it doesn't allow you to prove more about the encryption and you can't prove the published source matches reality.
Obviously there are business/brand reasons as well, but I think Telegram is being honest here.
[1] https://github.com/signalapp/Signal-Server