Yes, exactly. I think the difficulty is that the invariance theorem talks about any two fixed languages - then we can bound the difference between them - but th

Yes, exactly. I think the difficulty is that the invariance theorem talks about any two fixed languages - then we can bound the difference between them - but the article here talks about all possible languages. If you consider them all at once, things do seem peculiar, but it's meaningless: Considering any one is sufficient, and the invariance theorem justifies that by showing that no other one would be better.