From the people who had no problems naming their product identical to the colloquial name of the oldest maintained GNU/Linux distribution, thereby confusing the hell out of actual technical people for the longest time with press releases.
Also, Bob Dobbs called...
(Edit: you can take away my upticks, but you will never take away my slack. "Hacker" news my ass.)
I bet the Slack top brass just told legal to throw the book at the guy because of the name, and so they padded the letter with everything they could find, including weak technical arguments that they don't even enforce on others.
(I agree on the confusion with Slackware btw, it took me a while to realize they had no connection whatsover with Volkerding or the distro.)
Frankly the origin of Slack's name has nothing to do with Slackware Linux (or Linux in general) so I'm having difficulty following your grievance here.
Now if "Slack" means the pretty protocol-breaking irc-client to you, that's perfectly fine. But if some party starts threatening someone with 'That's not what "Slack" means! We're the authoritative source on the meaning of "Slack"', a hacker should resists. Slack is for everybody. Many may have forgotten, but it's not VC-money that makes the Internet go round, it's Slack.
To quote the prophet:
"If someone is making money off ``Slack'', it better be me."
"Those who cannot remember the past are doomed to trade their precious Slack for worthless trinkets."
"The Slack that can be described is not true Slack."
Okay but they didn't do that. Slack is concerned about someone using their name in relation to third party, unaffiliated software which is obviously related to their own software. Slack the messaging company doesn't care about the use of Slack outside of that niche.
If you wrote a browser extension called "BetterSlack" that emulated Slackware in your browser, Slack wouldn't care. If you wrote a browser extension called "DontSlackOff!" that automatically restricted the time you could procrastinate on Hacker News or reddit, Slack wouldn't care.
Slack cares about its brand, not being the canonical authority on an English word use of that word in products. The idea that Slack cares about uses of the word that don't have anything to do with its own software seems like a willful misinterpretation of the point. This is a pretty mundane legality and has nothing to do with hacker culture or "resisting" anything.