Did the slack warning not tell you the number of people it was going to ping? At least nowadays it says something like "You're going to ping X people in Y timezones".
As more context, I do remember thinking, "that X number seems higher than I would expect, but maybe we had a lot of folks who signed up for the event that didn't show." I worked at a small company so the size of the company was on the same order of magnitude as the number of invitees to the event (~100). I explained away the Y time zones because I knew some people traveled internationally. I was also operating on very little sleep, so that probably didn't help.