Countries by registered Hockey Players in 2018/19
1. Canada - 621,026
2. US - 567,908
3. Czech Republic - 121,613
4. Russia - 112,236
5. Finland - 64,641
6. Sweden - 55,431
7. Switzerland - 27,867
8. France - 21,667
9. Germany - 21,340
10. Japan - 18,837
Goes on to list another 10 countries - Ice hockey players in the world | Statista
The US has wayyyy more registered hockey players than the Czech Republic, Russia, Finand, Sweden and other combined. Countries where Hockey is definitely not a niche sport.
There is simply no way that Hockey is a niche sport in the US, you may think of it as such but the reality is there are 24 NHL teams with 20k seat arenas to host them. Its a big business, while it might not be as big as the NFL or MLB or even NBA, I think the stats clearly suggest that Hockey is well past the cutoff of being considered a "niche sport" in the US.
An example of a niche sport in the US would be Vollyball or Squash, not Hockey.
A more interesting comparison would be registered players/divided by people <55 or something like that. Sure, the U.S. has 5x the hockey players than the Czechs. But there's over 30x more Americans than Czechs. So I'd say that hockey is definitely more popular in the Czech Republic than in America.
Just going by % of population that plays hockey:
Canada - 1.65%
USA - 0.17%
Czech Republic - 1.14%
Sweden - 0.55%
Russia - 0.08%
Finland - 1.16%
So whether you want to call hockey a niche sport in the U.S., Americans aren't all that interested in playing. It's definitely more of a niche sport in America than in the Czech Republic, Finland, or Sweden.