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.
Please Watch/Share this video so it gets shown in Hollywood.