An argument for semantic externalism

I am not actually convinced by this argument. But let's go.

1. If it is possible that whether an utterance expresses a proposition depends on what happens at the same time in distant places in the universe, then semantic externalism is true.
2. It is possible that whether an utterance expresses a proposition depends on what happens at the same time in distant places in the universe.
3. Therefore, semantic externalism is true.

I am not going to defend (1). It's fairly plausible, though I think it's actually a weak point. I want to defend (2). Suppose Fred and Janine are in distant places in the universe, and speak simultaneously. Fred says:

4. The sentence Janine is now uttering is true.

Janine says:

5. The sentence Fred is now uttering is not true.

It cannot be that both of Janine's and Fred's utterances succeed in expressing a proposition. For if they do, then if what Janine says is true, it is not true, and if what Janine says is not true, it is true, which is a contradiction. But in a world where Fred instead says:

6. 2+2=5.

and Janine still says (5), what Janine utters does express a proposition—indeed, a true one. And in a world where Fred still says (4) but Janine says (6), Fred's utterance expresses a false proposition. So it must be that whether Janine's utterance expresses a proposition depends on what Fred says far away, or that whether Fred's utterance expresses a proposition depends on what Janine says far away. Either option is sufficient to yield (1).