They probably all got worms, or maybe even had worms to begin with. The freshwater clam lifecycle is one of the more complicated life cycles and interesting.
One of the colonel's enterprises was a stagecoach line to take salmon from Seattle to New York on ice in 7 days. So, anything's possible.
Smoked salmon also makes very tasty sushi if fresh isn't available. Tamago is an option that might appeal to a reluctant North Dakotan.
Oh, and salmon can be found in some waters of North Dakota; I don't know if they're landlocked or what, but that's an option too.
https://en.m.wikipedia.org/wiki/Swedish_emigration_to_the_Un...
I don't think you can even call it seafood.
Surströmming beats lutefisk for that title.
[1] https://en.wikipedia.org/wiki/Fesikh and pronounced like this "Fisíj" borrowing from Spanish
Could be biased, I do most of my fishing in the Colorado rockies and have not had problems with worms there.
Sure, they're not going to find it locally available, but I don't see why it would be impossible for someone wealthy to get what they wanted.
As someone else mentioned, it was probably tamago, or maybe beef. Or maybe even local fish but pickled, which would have gone with the vinegar rice.
Or maybe they used local fish raw and just risked getting worms.
It seems entirely reasonable to me that a person in North Dakota could get fish delivered from the coasts, on ice, in a couple of days. Now, did they? I honestly have no idea.
Heck, half of the point of the article seemed to be that we seriously underestimate the people (Americans, in this case), of the 19th and early 20th centuries.