When the request is sent Chrome doesn't know the content type of the response, so it can't know that what it's requesting is an already-encrypted file.

Who said anything about encryption?

Good question. Why doesn't chrome decompress it when their header suggests they will? Yes it's unneeded and discouraged, but if developers do send gzipped content such as images (which as we all (including the blogger) know, they definitely shouldn't!) surely chrome should just go ahead and decompress it as normal?

I'm not misunderstanding anything. Your very last point contains:

"We still don’t quite understand why nobody sends gzip’ed images"

Quite right, I have corrected that part of the conclusion, my question referred to the latter part, which I paraphrased above.