It was a different time back then, hardware vendors had to sign exclusivity agreements, they were in general big bullies back in the day.
I am very very thankful that our industry has evolved beyond this type of character.
...
UPDATE: For those of you who missed the subtlety (which was a surprising lot of you) I was quite polite to this guy on the phone. """
It seems to me that in the phone conversation, ESR made it pretty clear that he was not seriously considering it, and that he was going to reply in a strange manner.
I think he has a weird sense of humor, for sure, but I don't think he was particularly bullying that particular recruiter -- he was putting on a show of bullying Microsoft, because that was both his thing (as an early Open Source proponent) and fashionable at the time.
Currently, his hobby appears to be posting wingnut craziness on his website.
The Art of Unix Programming is a great read.
Just make sure you don't read his blog.