For context, one of the original reports of this was from (I believe) an Iranian-American with US Citizenship living in the US:

https://twitter.com/aaomidi/status/1075621119028314112

You might read that and infer that Slack is somehow tracking the national origin, ethnicity, or race of all of its users, which would be much fishier behavior than IP-based blocking. They're explicitly saying that they don't do that and that they don't have that information.

I see it as a technically true statement: they blocked users who looked like they were connecting from within an embargoed nation, regardless of these users' actual nationality.

i am an american national and i go to canada. my access to netflix changes for reasons that have nothing to do with my nationality (what passport i have), and everything to do with what nation i’m in.

They likely blocked by whether you ever signed in from a blocked nation. That's not necessarily the same as blocking any users who would qualify as exporting software to a blocked nation, which is also not the same as blocking everyone whose nationality matches said nation.

They don't care about the nationality of the person, just the location of the IP address.

I think they meant nationality as in the person's nationality, they were blocking solely based on an IP based location.

They were blocking everyone in that nation, regardless of nationality.