This would be great if Nintendo was genuinely concerned about encouraging the usage of a hostile platform. Sadly odds are that it cares about its brand way more than "those things" playing their games.
Nintendo didn't provide one [reason], but it's likely due to X's increased API costs
I don't think so. Even with the abusive costs the price would be rather small for Nintendo, in comparison with the advertisement of its games in Twitter. I think that it's mostly a "eeeeew, Nintendo has X integration? Nintendo must be fascist."