There might not be copyright infringement involved, so "stealing" is an unnecessary assumption (on top of being a misleading term for copyright infringement). Since Nintendo isn't the only source for games that run on Nintendo consoles, Nintendo might not get money from the self-proclaimed boycotter. But since there's no guarantee that Nintendo won't get money from the self-proclaimed either, I can understand why people would view the following scenario as cognitively dissonant:

  A person only buys physical copies of non-Nintendo-made games which run on Nintendo devices from people reselling legally-distributed copies (legal in the US with the first-sale doctrine, which unfortunately doesn't apply to digital copies). The person plays those games by ripping the files and running them on third-party emulators. The person publicly states that they boycott Nintendo the company and that they play non-Nintendo-games made for Nintendo consoles. Resellers might or might not spend their money on more Nintendo games.

By the way, iamunr didn't claim to boycott Nintendo. Perhaps iamunr buys Nintendo games but plays them on emulators. throwaway48r7r didn't provide any information about whether they'll continue playing non-Nintendo-made games for Nintendo consoles.