Well, he's half right. PS5 and Series X are not going to slow down Switch at all. On the other hand, the former two consoles are going to sell sluggishly for the first few years. Switch is going to affect PS5 and Series X just like Wii affected the sales of PS3 and XBox360. The Wii sold great for a few years and the other two were sluggish at first. It didn't have a big effect on their lifetime sales. Instead is was mostly a delaying effect on their sales. Switch will have the same delaying effect on PS5 and Series X.
That's not what happened.
With the exception of the launch window, due to shortages, 360 was outpacing its predecessor the entire time.
Xbox vs 360
Launch-1.5m vs 1.2m
Year 1-6.5m vs 6.8m
Year 2-5.7m vs 7.9m
Year 3-6.2m vs 10.9m
Year 4-4.1m vs 10.2m
Basically they tied for the first year (when Wii wasnt out yet) then 360 started outpacing it.
As for PS3, Wii had little to do with its sluggish sales, it was the ridiculously high price, Blu-Ray not being as big of deal as DVD and the loss of a ton of exclusives.
Wii was like Switch, great sales that were independent of the other consoles and complemented rather than competed with them.