It’s easy enough that children can do it. Cycling on hard-packed snow is not particularly hard, less so than sand, I’d say comparable to gravel. Ice is a problem.
Convenience is relative. Sure, cycling in winter requires a good pair of mittens, and a warm hat, but all in all, it seems that a lot of people prefer it in Oulu over driving.
And the thing here is that it’s not the weather that’s stopping people from cycling. It’s shitty maintenance of the cycling network. Slush and ice, being forced into the same space as cars.
And that’s the newsworthy part - how does Oulu manage to maintain a cycling network in winter where most other cities fail. The answers include things such as priorities, dedication, investment. Oulu dedicates resources to the problem. Most towns don’t because “no one cycles in winter” - which then becomes a self-reinforcing prophecy.
It's fine as long as bike paths are physically separated from cars and are maintained to the same standard as roads. My college had a network of pedestrian/bike paths that they plowed in the winter, and I and many others had no problem biking year-round.
There's a difference between riding on hard pack with a bike, and driving a car over it. I have no idea why you think an experiment with hard-pack snow on automotive roads is in anyway indicative of what works on cycle lanes.
In Reykjavik they used sand at first because pedestrians prefer that but that is pretty dangerous for cycling so they use salt on designated cycling paths and sand everywhere else.
It depends.. salt works down to -18C on cycle paths as long as you maintain it snow free and do not have big puddles of water. Brushing the snow away down to the asphalt is my preferred surface condition for cycling.
Amazingly, there is a city almost as dominated by cyclists in Finland (don't remember how it's called atm). The city is plowing and de-icing all the bike paths on regular basis. They're also well-lit, as people's commutes during winter months are often in the dark.
It’s not just Oulu, though: any city with students in the North has a lot of bikes, Umeå in Sweden has 40k students almost all on bikes; Vaasa (opposite Umeå, in Finland) also has a lot of bikes. Uppsala, Stockholm, Malmö, Göteborg, Eppo, all have a lot of bikes—and just as many in winter and summer.
Another interesting fact is that at least some of the cycle lane markings are projected rather than painted because otherwise they would be covered by the snow.
Not that often anymore (most winters there's at most one or two weeks with snow cover), but when it does, in many municipalities cycling paths get priority for snow removal.
