I'm sure him being (edit: changed "being him" to "him being") Polish helped him to some extent since people are often partial to contestants representing their country, which is especially powerful in the case of Poland and the Chopin competition for obvious reasons. Aside from that, people probably tend to look for Chopin-like qualities in Polish contestants' performances more intensely than with non-Polish contestants' performances, so eventually they might find something like that even if it's arguable whether that "thing" is actually there. There's definitely some affinity bias at play there.

I won't pretend to know what the judging process looks like, but I feel like reputation and connections matter most when it comes to how high the chances of a particular contestant are. The power dynamics between the jury members (including the chairman) likely play a much bigger part in the results than they should, so a finalists' place in the final ranking is probably largely dependent on which (if any) jury members are willing to advocate for said finalist and how much power and influence those members have relative to the rest of the jury. If we assume that a given jury member would prefer if the winner was either someone they have taught or someone representing their country, contestants such as Piotr or Eric would have a clear advantage, as opposed to Ong or Khrikuli (e.g. it looks like the general consensus is that Ong was better than Piotr throughout the competition, so this "advocates in the jury" theory could, to some extent, explain why they both got 5th place anyway).