Haven't been there that late myself on Sundays, but I have to admit that this was exactly what I expected... It can be hard to find a dinner restaurant even in downtown Helsinki on Sundays, not to mention Sunday evenings. Naturally you can get food from multiple places (mainly the S-chain outlets though) also with decent quality, but when in a situation to recommend something to foreigners or to celebrate with your loved ones: to name five can be tough. Hopefully the new restaurant world will make difference in this sence.

I would suppose though that something could have been available at the Hilton, since at least when they opened it was said that they serve breakfast 24/7. Would expect them to have someone present also for roomservice. This naturally requires exiting the terminal building, but at least wouldn't personally mind to get some non-AC air between flights if there's just enough time.