Astro and Fresh were both inspired by the islands idea which was iirc coined by an Etsy frontend architect and further elaborated on by the creator of Preact.

My understanding is that Astro is able to more-or-less take a component from any combo of popular frameworks and render it, whereas Fresh is currently limited to just Preact via Deno. I think the limitation is to optimize for not needing a build step, and not having to tweak the frameworks themselves like Astro does (did?).

I'm not affiliated; I've just looked at both tools before.