The response was fine, in the same tone as the original request even if the first one was offensive by accident. A kind, functioning human would be puzzled at the slightly blunt response and re-evaluate the whole conversation.
They would not go to a private channel, knowing that they would get blasted for such a response in a public forum, and get even more offensive. Plus, any non-native that has an advanced enough understanding of English to use the phrase 'thinly veiled extortion' can also control the tone of their requests. No need to defend IBM here.
You can hardly say that they switched to a private channel because they knew "that they would get blasted for such a response in a public forum" when the comment they were responding to was the one that suggested the switch to email.
The comment suggested email for payment, not an extortion accusation (i know accusation is bit too much of a strong word, i couldnt find anything better)
Plus, they did get blasted just because the email was made public. To me, that shows that they wanted a more private channel.
They would not go to a private channel, knowing that they would get blasted for such a response in a public forum, and get even more offensive. Plus, any non-native that has an advanced enough understanding of English to use the phrase 'thinly veiled extortion' can also control the tone of their requests. No need to defend IBM here.