Its CO2, not propane.

You don’t have the whole context. The relevance of it being interstellar is that, at the speeds it was travelling, it should have vaporized completely when entering out atmosphere. The fact that pieces survived reentry is anomalous, as it indicates that it must have been made from alloys not naturally present in any other object that we’ve seen entering out atmosphere from outer space.

If those alloys are natural (but never seen before) or artificial (and so created by some other intelligence), that’s the question here.