Why has Signal even enabled caching for those URLs? The most common case is going to be that the attachment is downloaded once, and that's it.

I would even expect that Signal wouldn't allow you to download it more than once, and would immediately delete it after the first successful download. Well, ok, maybe the client fails mid-way through, so allow some grace period for a re-download. But I can't imagine that would be the common case either, and so disabling caching on their CDN would fix this issue, and hopefully not increase their costs much.

At any rate, "deanonymization" is a bit clickbaity here. Narrowing someone's location to within 250 miles or so isn't great, but it doesn't deanonymize them.

Edit: I didn't think about the case where an attachment is sent to a group chat, where multiple people will be downloading it. But in that case wouldn't the attachment be encrypted individually for each person in the group? I'm not sure how this works, of course.