Simple: Nintendo is also a hardware manufacturer, and thus a direct competitor to Sony and MS. The same it would have been stupid to not factor the PS4 and X One when the Switch launched, it would be stupid not to include the Switch into the equation, because it WILL be stealing sales away from MS and Sony.
It's like asking which kind of chocolate is better, but leaving milk chocolate out of the question for reasons.
But the Switch is not launching this year. The OP and the poll only concern the two major platforms launching this 2020 holiday season. It asks if PS5 will outsell its rival (singular). People always like to bring up Switch in these cases just to brag. The Switch is irrelevant to the question which will sell better at launch, PS5 or Xbox.
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.