Playing Devil's advocate, the "Rival" could mean either Nintendo or MS. If it were up to me, Sony actual rival is Nintendo, but that's more of an observation from me than anything else.
But on a serious note: If the question was actually " What system is going to be more attractive/have better games/be better: Sony or MS" then yeah, I would agree with you that Nintendo has nothing to do with said discussion, simply because it's more about the quality of the hardware/games, and the Switch selling like hotcakes right now will (probably) not influence that.
But that's not the case. The discussion is about sales, and it has nothing to do with bragging about Nintendo's success(That didn't stop people " bringing up" the PS4 and XOne sales up when the Switch launched though...), it's simply that Sony and MS dosen't exist in a vacuum. Any hardware sales discussion needs to involve Nintendo, Sony and Microsoft, otherwise it is meaningless. You are having an discussing with half the information pertaining to it.It would be the same situation with my chocolate example.
People like to ignore Nintendo as if it existed in another dimension when they are winning. It's stupid. And I'll always treat it as stupid. If it is a discussion that I feel like there is someone/something missing, I'll bring it up.
Then by all means create a thread asking which will sell more this holiday season: Switch, PS5, or Xbox. As it stands Nintendo and the Switch have nothing to do with this thread or topic.
Please Watch/Share this video so it gets shown in Hollywood.