Sweden didn't mandate strict measures.

Plenty of folks still took them on their own volition.

No mask, public transports, restaurant, schools open, etc

doesn't matter if even 20% of the population (which is a very high estimate) use some of these measures. You can't explain that the evolution is exactly the same as a Norway...

You can, though.

If you get a disease's R0 below one, it goes away.

Enough people changed their behavior in both countries to make flu go away, because it no longer spread quickly enough to sustain itself.