It is not "considered" international; it IS international. Norway and Sweden are two different countries, wholly independent of each other. The Nordic Passport union allowing passportless travel between the countries does not change that, any more than the Schengen treaty makes Finland and Spain the same country since you can travel passportless between them. Actually, travel between Norway and Sweden is more international than travel between Finland and Spain or any other EU-country, because Norway is outside the EU. There are customs controls between the countries with stricter limits than within the EU, and travelling between Norway and Sweden even entitles one to shop duty free. Sorry for the rant, but independant European kingdoms should not be considered the equivalent of states in the US.
Yes, I think the rant is a bit more thhan needed. It's not unheard of for international flights that do not require passport checks to fly from domestic terminals. In the US, I still sometimes see an international flight depart from a domestic gate. So it's not unreasonable to wonder if a flight from Norway to Sweden would always land and take off from an international terminal, espescially when a passport check is not required to cross the border. If the flight was permitted to land at a domestic gate at ARN, then connecting to a US bound flight leaving Scnadanavia would be a big problem.