But they don't know that. They have IP addresses. And they do geolocation. But IP-based geolocation isn't reliable enough for that.
They have information from which they conclude that; this information is fallible and conclusions are not certain, but that's true of virtually all “knowledge” about the material world.
Iran does not recognize changes in citizenship.
The U.S. government doesn't care what Iran recognizes. The correct quotation would be "anyone who is legally an Iranian citizen in the eyes of the U.S. government."
When an Iranian citizen becomes a naturalized U.S. citizen, their Iranian citizenship is no longer valid in the U.S.
There is a limited number of nations that have dual citizenship agreements with the United States, and Iran isn't one of them.
(a) the objectives of laws prohibiting large commercial flows to sanctioned countries and
(b) the objectives of laws encouraging many tiny information exchanges on the internet, taking place outside of sanctioned countries
Is it worth burning down Internet commerce in the hope of catching a few individuals? Did Congress intend to create a Do-Not-Speak list, or one such list for every Internet company? Should Internet UGC platforms now relocate outside the USA, when Congress is also encouraging companies to repatriate assets to the USA?
What's happening here isn't an attempt to catch a few individuals. The goal is to put pressure on the entire government of Iran. This is done by making doing business hard for large businesses, as well as individuals, so that pressure to change is put on the Iranian government from above (businesses) and below (the people).
Penalties for noncompliance are stiff and there are no safe harbour provisions.
(IANAL but I have implemented systems to check OFAC lists at other companies and seen it result in similar situations)