Nuxt is a server-side JavaScript framework with a complete HTTP application stack, built on top of Nitro (which includes an even larger suite of capabilities) and H3.
The idea that Astro and Nuxt are only alike because they "print markup" is factually incorrect. Astro is designed to give you the same flexibility as Nuxt when it comes to building dynamic, server-rendered sites. One can even use React or Vue within Astro. You can create API endpoints, use SSR, and even define backend functions with type-safety. The migration guide from Nuxt to Astro makes it clear that both frameworks support similar workflows, from file-based routing to dynamic data fetching. Astro’s islands architecture[0] just means you get more control over how and when interactivity is delivered, but under the hood, it’s every bit as much a server-side javascript framework as Nuxt.
[0] https://docs.astro.build/en/concepts/islands/#server-islands