1% mask usage in Sweden and we also have extremely low flu numbers.

Interesting. Do people still associate indoors in large numbers, or is that unchanged as well? As in, could behavioral change be a driver?

I also wonder, with far less numerical opportunity to spread and mutate, if the flu wasn't able to "grow" a good candidate with good evolutionary fitness this year, so distancing and mask usage affect countries even where they aren't practiced.

No, we have other restrictions and recommendations resulting in various degrees of social distancing. Definitely less than countries that did lockdowns but it's probably as you say a combination of our own measures and other countries' more rigid restrictions (including masks) killing off the international transmission routes. So we should be thankful that other countries took this more seriously if that's the case.