What is hydrating the client?
This honestly reads like satire. It sounds like something on the sarcastic VanillaJS homepage.
This gibberish being the second bullet point in a list of core features is a huge turn off. I know it’s tongue in cheek, but still
I think it's mostly like a tongue in cheek acknowledgement of how everyone in big techs is fighting so hard for a promotion that they aggressively brand the heck out of every library, exploit, and "new tech" they scheme up, even if the thing they're mentioning is using weird language for a concept that isn't new.
* Island refers to this https://jasonformat.com/islands-architecture/
* Island based hydration is a form of partial hydration where the boundary is the component islands.