I don't think it necessarily has to be surprising to be interesting. If it came from another solar system, it's traveled more than 4 light years (obviously much slower than light speed) to get here. That's a long time, and our solar system is a very small target, so the odds are (literally) astronomical.

There's a lot we don't know about the space between solar systems or the space outside of our own heliosphere. This is evidenced by our lack of understanding of when (or if) Voyager actually left our solar system. We just don't know enough to say. Having the opportunity to see something that came from outside our solar system is a good thing for science.

It's not particularly surprising, but this is (probably) only the second such object we've actually seen, which makes it very interesting.

The first such object, ʻOumuamua, was both interesting (because it was the first one we saw) and surprising (because of its apparent odd shape as indicated by its observed light curve).

A lot of rocks get ejected from solar systems. Current dynamical models suggest there are as many as 3 of these sorts of objects passing through on closest approach every day. We just aren’t very well equipped to detect them.

You can assume but you don't know until you actually observe them. It's no longer an assumption.