<script>
import fs from 'fs'

export const preload = () => {
  return {
    data: {
       content: fs.readFileSync('foo.md', 'utf8')
    }
  }
}
</script>

<script setup>
// ...
</script>

fs will be unexpectedly added to setup() function even if it's not imported in <script setup> but a regular <script> block, this is a known issue of @vue/compiler-sfc.

The workaround is to not to use preload with the <script setup> block.