A better conclusion is that "This shows that launching a console late into the generation will not negatively impact the sales" 

I remember a couple people saying before Switch launched that it will do bad because people already own a PS4 or Xbox One. But as we see here, people don't mind owning 2 consoles.

Eh, didn't Bethesda say a few days ago that all their games were successfull on Switch?

I know I have both a Switch and a PS4 Pro. The Pro is my main console for Sony exclusives and multiplats, while the Switch is my secondary for Nintendo titles.

Wolfeinstein 2 didn't launch on switch last year. It came to PS4 and xbox but launched the same day as Mario Odyssey. 

It's not really that surprising: the target audience is the same; and despite the Switch's huge success, PS4 and XB1 didn't go down.
It was "obvious" that those consumers were massively buying a Switch.

There are two things that i think should be taken into consideration:
Switch was lucky to come out at the time it did (it had 3 years of a userbase to take advantage off and, despite Switch's success, it remains to be seen if it can go from secondary console to primary console to that audience. If achieves that, it will be a bigger win that whatever it sells in it's lifetime.

The irony is that so many PS4 owners thought they needed the Switch to play games 

Gamers buy gaming machines.

Strangely enough, there are games that I buy for the Switch instead of my more powerful PS4 Pro and XBO X. There are even a handful of games that I already have for those consoles but I buy them again for the Switch because portability is just that important to me.

Not always. Only really since 7th gen with the Wii. In 6th gen, both Xbox and Gamecube were secondary consoles to PS2, but both had pretty major primary fanbases as well. In the 5th gen, N64 competed directly with PS1, and lost. It wasn't a secondary console, just second place in the primary console battle. In 4th gen, SNES WAS the primary console, beating Genesis handily. In 3rd Gen, the NES was king, crushing everything, selling more than the rest of the consoles combined several times over. And then the Nintendo portables, well, they were always the primary system.

I would love to get figures for what proportion of ps4, xbox and pc gamers own other secondary consoles. I have no doubt that the figure for the Switch is going to be higher than the others.

or could it mean that COD Black Ops 4 and Red Dead Redemption 2 flop...

But seriously, even if Mario Party had a slow launch because of those games, Nintendo titles tend to have long legs, and so the most those other titles would do is slow the initial sales, but minimal impact long term.