First off, this isn't a debate. Secondly, this completely ignores the use of tone in communication.
People get defensive when it appears they're being attacked. The "if you'd have asked nicely" response is, in my experience, the best response to give to a random being an asshole. Either they'll double down (confirming your suspicions), or they'll realize they came across wrong and rephrase.
An argument from a legal department is a threat. And if it's not actually a threat, it's certainly a very significant perceived threat.
And lawyers and society definitely understand that.
The letter itself wasn't terrible, but it definitely contained a threat, and claiming it is a tone argument denies that.
Also, do you have a reference to PG's "hierarchy of disagreement"? Sounds interesting.