Time setting is immaterial. Steampunk is science fiction set in the past. And you can have Space Opera set in the past (it has been done many times). Just the typical convention is to set it in our future when we have higher tech (or lower in the case of post apocalyptic SF).
I’m actually not sure if I would consider Steampunk science fiction either. But as someone else has stated, I guess it’s all about what definition of science fiction you subscribe too.
Another way to look at this question though. If you were to make seperate lists of the greatest sci-fi movies ever and the greatest fantasy movies ever, which list would you put Star Wars on?
Looking at this way I realize that I would actually put it on the sci-fi list. So even though I initially felt it was more fantasy, I now realize that the sci-fi elements are actually stronger for me.
Wow. I actually changed my mind about this whole thing just now while typing this post. Well… they say a mind that can change is a healthy one! 😃