I think you underestimate the level of national pride that players (and especially ones from some countries) take in representing their nations. Obviously he would like to play for the Cup, but he's at best the 9th defenseman in playing priority, so that's pretty unlikely. At that point I think it's super reasonable to go to the WC, especially if you know you're one of their best defenseman and could meaningfully improve their team.
He may not, of course, for a myriad of reasons but I wouldn't write it off so easily.