That XKCD assumes the person actually got the message, in which case a month is fine.
Sending an email doesn't cut it on this short of a timeline.
The really weird thing is they had seven months to tell people about this deadline, and waited until six of them had passed.
Why do you assume the decision was made upon acquisition? Things change.
Everyone who follows tech startups saw this one coming. This was a clear acquihire. My company was in the process of acquihiring. We knew from a mile away that they didn't give a crap about continuing the development our product, despite all their promises. We ended up declining (none of us wanted to move to the bay area).
Oh they might have procrastinated but the decision should have been made around that time. It's not an excuse to say you wasted so much time deciding how to shut down the service that people need to get out out now now now.
You give them one more user, which is worth a lot when a software company negociates its value before a buyout.