I don't think a "he used to be worse" defense is a takeaway from the above. The comment mentions the past to highlight it's not like previous rants people may assume it might be like to encourage giving it a read. Separate explanations were offered for why they believe the message was a reasonable response in its own light.
I agree the last 2-3 paragraphs could be better but I disagree that's the bar for what's professional, let alone what is reasonable to stand out out of a month's worth of emails. It's also hard to square that message being so horrible by nature of being shitty and just to make Linus feel better - to me, these are more pointed and more personally directed statements than the supposedly problematic discourse they're about (though still relatively mild overall).
We aren't all perfect in communication all the time, even the most ideal professionals. A large part of good discourse is accepting reasonable amounts of people being human and de-escalating smaller perceived slights naturally before declaring the conversation can't be stood for. Jumping at the latter can cause well meaning and well received folks to feel bullied, which is the exact opposite to the goal of trying to keep a professional air to discussion. Hell, some of the times I've been more colorfully told to look in the mirror were the best things which could have been said to me at the time (even if they might not be reasonable things to have heard at other times).