The post seems to claim that there is no drawbacks others approach with the 0 bytes of js, and link to a blog post [1] that absolutely tells nothing how to properly manage back/front state and rendering consistency between the rendering and the «islands» on the client.
Without having built in the "Island Architecture", it looks exactly like a page of old where you serve an entire page, accept form submissions, return the updated page, and make small elements interactive (islands). These islands contain only minimal local state and global updates happen through the aforementioned form submissions.
The difference being it's all one isomorphic (?) React-y Deno codebase instead of the tangled PHP+jQuery mess.
[1] https://jasonformat.com/islands-architecture