It sounds as though you have access to data that the rest of people on HN do not. Would you care to share your source? Are you certain he would not have been banned by Slack if he were not an Iran citizen (just Canadian) but would have (possibly) used Slack when travelling to Iran?
We're both adults here and I assume we both can realize that splitting these hairs doesn't make my statement that banning everyone that's ever logged on Iran from slack is, in effect, a ban on Iranian ethnic users?
So no, a ban based on signing in from a specific locale has zero to do with ethnicity or race.
On the other, the author of the thread is accusing Slack of profiling him racially. What he's implying is that a human proceeded to stalk him around the Internet, on social networks, etc to check if he is an actual Iranian and ban him based on that. And that is a very, very grave accusation.
Banning anyone who has ever used Farsi in Slack, now that would be a bit closer to banning for ethnicity.