Why would this work:
const PostComponent = ({post}) => (
<span>
<div>{post.text}</div>
</span>
)
export default PostComponent;
And not this:
export default PostComponent = ({post}) => (
<span>
<div>{post.text}</div>
</span>
)
The later says the component is undefined when imported.

PostComponentis not defined. And you can't define and export at once with default export.export default function({post}) { ... }is possible however.