You can argue that there's no strict comparison between Telegram and WhatsApp, e.g., if you trust Telegram more than WhatsApp, then you'd rather have both your metadata and your data going through Telegram's servers rather than only your metadata through WhatsApp's servers.
But you can't say that a non-E2EE messenger (by default) gives you better security guarantees than WhatsApp which is E2EE by default.
Apparently the main argument is open-source vs closed-source. Links between being open-source and being secure have also been debunked many times already. It's strange because the author mentions that server software being open-source cannot give you more confidence that it's secure, while apparently using an open-source app you didn't build yourself does.