JSX is in global namespace. That's why it is added here.

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/7d19d6ff87c33d30e2c81b44e5763f7d0453233f/types/react/index.d.ts#L2943-L2944

Do you know at what point this would come "in scope" ? I guess if it were an issue, the TypeScript build would be complaining about it?

The members of JSX namespace are all types or interfaces. They're imported when we import react. So, TypeScript will only complain we use JSX elements before import something from 'react' like:

/**
* @return {JSX.Element} a component
*/
function f() {}

const React = require('react');

I'm not sure it would be even an error if hoisting happens. It's also against our coding convention.

In conclusion, it's almost impossible to happen.

One of the worries I might have is that for createElement in particular, we automate this via a Babel transform, so that the source doesn't actually include an import statement.

See: https://github.com/WordPress/gutenberg/tree/master/packages/babel-plugin-import-jsx-pragma

I've tested by adding a new file without importing React:

// a11y/src/index.js

export { X } from './x';

// a11y/src/x.js

/**
 * Test if it works without TypeScript complaining.
 *
 * @return {JSX.Element} React component
 */
export const X = () => <div />;

It works without any complaints. We're good to go.