`signal` key has defined its key as optional, not necessarily the value even though this happens implicitly.  No key means no value which translates to `undefined`:

```ts
    signal?: AbortSignal | null;
```

To support `exactOptionalPropertyTypes` it would have to be defined as: 
```ts
    signal?: AbortSignal | null | undefined;
```

While you could say the key isn't optional (removing `?`), which then requires an explicit `undefined` value: 
```ts
    signal: AbortSignal | null | undefined;
```

That would require an explicit undefined being passed. 

Given this key is part of TS definitions and module augmentation is overkill, just "oring" from undefined to an expected value (in this case `null`) solves the problem.