Speaking as someone who is at least 200 times more likely than the general gmail-using public to receive mail from completely normal, mainstream Chinese email addresses... I'm still mad at gmail for just assuming mail from China must be spam. It's not spam!
(There's been improvement - for example, recently I received mail from someone I'd corresponded with in the past, and it wasn't initially marked as spam. Gmail used to be more aggressive than that, such that it would be marked as spam unless it was a direct reply to an email I had sent.)
(...for extra irony, that recent message was "I'm stuck in England and can't get home without a few thousand dollars". Her account had been hacked.)
Value judgements aside, that's not what I'm postulating. It's more like "unknown" mail servers (meaning any mail server with low enough volume that Gmail doesn't have an opinion about it yet).
What about using it as one of many criteria (maybe whether the email seems to contain gibberish being another, which patch files might be classified as).
Out of all the people in the world who regularly get mailed "gibberish" patchfiles, and not only fail to mark them as spam, but continue to interact regularly with the senders - do you not think it's reasonable to assume Gmail might notice Linus has been doing this for two or three decades?
I wondr what other forms of "gibberish" Gmail classifys as spam? GPG encrypted mail? Mail containing public ssl keys or CSRs? ANy foreign language not regulalrly hard in Bro-ville, South Bay?
What about the possibility that 'vanity' email servers are more likely to have something non-obvious misconfigured? Maybe, for example, they send spam rejections back to the envelope from address (generating backscatter which looks like spam), rather than rejecting spam within the SMTP session?
Assuming non-gmail servers are spammers is a pretty epic fail.