Probably because it's a subject I'm interested in, and curious to know how I might be wrong. I'll note that I've learned at least as much on this site as I did completing a history degree (even considering all the books I read), and my professors were some very smart people too. Really, it's similar to the endless discussion on the American Civil War and the World Wars in parts of this site albeit a very niche subject. Personally, I don't recall discussing Alaskan farming more than 2 or 3 times in the past, and at least once it was in the context of Japanese colonisation which is different than OTL's subsidy-driven Alaskan farming. And 20th century Anglo farming vs. Meiji/alt-Japanese farming vs. indigenous farming are three different subjects.
Otherwise, the climate doesn't seem too different between Fairbanks and Delta, at least not compared to other agricultural regions. And a 20th century Anglo farmer has different concerns from an indigenous group which is farming for extra calories. A map of USDA soil types shows the two places have similar soil types.
We know lifestyle changes in indigenous America occurred, so if the coastal Alaskans have potato farming, I could see it potentially spreading inland and correspondingly resulting in a population expansion. Since you'd be tending potatoes for the 3-4 months of growing season, you wouldn't be able to hunt caribou as efficiently, which perhaps would mean you'd want to corral them more and maybe eventually domesticate them with all that entails. The expansion of agriculture happened like this in the northeastern United States/Maritimes, so it could happen in Alaska in a place like the Tanana Valley which is at the edge of potential land for farming.
It really isn't fair to call these alt-Indians a "more advanced" civilisation, but it's a civilisation which would likely be greater in numbers, even after the inevitable mass death from disease. No, it won't be "Tenochtitlan on the Yukon", but it would be more important than OTL.