By alien (as in the article’s headline, do the authors limit their claim to “from another star”, or do they actually mean that some other intelligence constructed it?
Nothing to do with intelligence at all. It's a comet, and a pretty ordinary one, not a spacecraft. What makes it alien is that it's not in orbit around the sun, it's just passing through.
It'd be much bigger news if it was constructed, I'd think.
Hey if I were observing humans, all my ships would look and scan like comets.
As we see from New Horizons, a flyby gives you very limited options for scanning. But hanging around requires eliminating nearly all the velocity you had to get there, which is infeasible.