os.DirFS implements io/fs.ReadFileFS, but returns the interface io/fs.FS, which means you need a type assertion to make use of it:

package main

import (
   "io/fs"
   "os"
)

func main() {
   b, err := os.DirFS("hello").(fs.ReadFileFS).ReadFile("hello.txt")
   if err != nil {
      panic(err)
   }
   os.Stdout.Write(b)
}

if we just had some concrete type that implemented the interfaces, it would remove the need for a type assertion:

package main

import "os"

type Directory string

func (Directory) ReadFile(string) ([]byte, error) {
   return nil, nil
}

func main() {
   b, err := Directory("hello").ReadFile("hello.txt")
   if err != nil {
      panic(err)
   }
   os.Stdout.Write(b)
}

ideally os.DirFS would have been a type rather than a function, but it seems its now too late for that. so I would propose adding a type that implements the same items as the return value from os.DirFS. this would also make discovery simpler, because the methods would be on the package site, rather than just in the notes of the DirFS function. I dont care what name is used, I just chose Directory to have something.