That's true, but in the Canadian example, presumably the government of Canada would extradite such a person to the US, while Russia obviously would not agree to

That's true, but in the Canadian example, presumably the government of Canada would extradite such a person to the US, while Russia obviously would not agree to extradite the culprit here to Poland to stand trial. Whether or not the government of Russia was initially involved, there's a pretty convincing argument to be made that a government harboring an individual who attacked another country makes it a government sanctioned action in at least some sense. Attacking infrastructure as a non-government entity can certainly be considered terrorism, but if the government backs you even after the fact, that feels a lot more like an act of war with plausible deniability. And in this case I think there might be a motivation for Poland to view it that way, since allowing Russian action against NATO countries invites more of the same, and Russia absolutely does not want to give NATO an excuse to retaliate in any way.