Type-safe updaters: non-assignability of unions

I seem to have encountered an issue with the new typesafe updaters. Here's the basic repro: https://github.com/Malien/typesafe-relay-updaters-repro

_Obligatory "using typescript, not flow" mention_

Assume we have a simple Todo app with todo groupings. Like this:
- ~~Make coffee~~
- Touch grass
- Buy groceries
- Tomatoes
- ~~Bread~~
- Work out

Let us have the following gql schema:
```graphql
type Query {
todos: [Todo!]!
}

interface Node {
id: ID!
}

union Todo = TodoItem | TodoGroup

type TodoItem implements Node {
id: ID!
title: String!
completed: Boolean!
}

type TodoGroup implements Node {
id: ID!
title: String!
todos: [TodoItem!]!
}

type Mutation {
createTodo(title: String!): TodoItem!
}
```

```graphql
fragment TodoList_assignable_todo on Todo @assignable {
__typename
}
```

given the query:

```graphql
query TodoListQuery {
todos {
...Todo_todo # Some component that renders the todo items/groups
...TodoList_assignable_todo
}
}
```

and the mutation

```graphql
mutation TodoListAddTodoMutation($title: String!) {
createTodo(title: $title) {
...Todo_todo
...TodoList_assignable_todo
}
}
```

the update of

```js
function updater(store, response) {
const newTodo = response?.createTodo;
if (!newTodo) return;

const { updatableData } = store.readUpdatableQuery(
graphql`
query TodoListUpdateQuery @updatable {
todos {
...TodoList_assignable_todo
}
}
`,
{},
);

updatableData.todos = [...todos, newTodo];
}
```

fails to typecheck, since the generated types from `TodoListAddTodoMutation` is:
```ts
export type TodoListAddTodoMutation$data = {
readonly createTodo: {
readonly __id: string;
readonly __isTodoList_assignable_todo?: "TodoItem";
readonly " $fragmentSpreads": FragmentRefs<"TodoList_assignable_todo" | "Todo_todo">;
};
};
```

`__isTodoList_assignable_todo` is optional.

This seems to be related to the sections of the docs ["where is guaranteed to implement an interface"](https://relay.dev/docs/guided-tour/updating-data/imperatively-modifying-linked-fields/#example-assigning-to-an-interface-when-the-source-is-guaranteed-to-implement-that-interface)/["where is not guaranteed to implement an interface"](https://relay.dev/docs/guided-tour/updating-data/imperatively-modifying-linked-fields/#example-assigning-to-an-interface-when-the-source-is-not-guaranteed-to-implement-that-interface), yet this seems like a missed case.

`TodoItem` (the return type of `createTodo`) is guaranteed to """implement""" union `Todo`. Yet this requires additional type-guard function (aka. validators). In the end the new typesafe updaters code is a lot more cumbersome to deal with, in comparison to the old unsafe variant. Not to mention all of the new concepts of assignability and updatability being thrown back at you.