The sportster is IMO, not a cruiser or a sport-bike (although it used to be/still is? used for flat-track racing).
Like a Triumph Bonneville, it’s just “a motorcycle” or to use an anachronistic title, a “roadster” (which they now call a “standard”). The terms cruiser (in this sense) and sport-bike, didn’t, I think, exist back in 1957 when the original was designed.
I tend to think of cruisers as having your feet out in front of you, the sportster has them mostly underneath you I believe (yes, know, that’s just one thing).