As you can tell from the comments, a lot of us first heard https://www.youtube.com/watch?v=FaSKD1Bd11M in the movie "Slap Shot." And I still got the answer wrong!
I think this is a case where, if you've grown up with knowledge of Saskatoon, you've probably never associated the name with anything other than the city (or berry) itself. It took me a while to understand the clue as well.
I would agree, Istanbul would need to take Tehran's place: The 16,000,738 for Tehran is including metro area, via Wikipedia - in the "official" JetPunk list it is listed as 8,895,947 for the city proper (https://www.jetpunk.com/info/capital-cities-by-population#list-of-all-capitals-by-population), while Istanbul has 15,514,128 in that category (on Wikipedia, as it is not listed on JetPunk, as it is not a capital). If not, a clarification regarding metro area would be helpful.
I think only 40% getting Port Sudan shows the hint should maybe be worded slightly differently. Something like, “It’s the largest port. In Sudan.” Without a reference to thinking. My guess is that that wording alone would lead to over 50%.
Tehran population: 16,000,378
(Especially given the nature of the clue!)
... oh, Saskatoon.