To be fair, the WiiU was a good system compared to the Wii, 360 and PS3. It wasnt marketed correctly and was expensive for what it was but ideally it laid the foundations for the Switch prodigy. 

I guess since he and The Grunt devs are among the very few singing praises for Xbox that must be true, the only people that didn't sell out to Sony right?

I doubt he was talking about hardware, moreover the concept. This was at a time people thought dual screen play was a good idea for consoles. A lot of video game guys thought this was the way forward but yet hardly third party supported the console largely due to the disparity in specs. The dual screen aspect also was good in theory and at the time but in practice it wasn't necessary. DS was the tech in its era, but then everyone got over it.

And wouldn't be too sure about that. Nintendo are smart enough to know that releasing a traditional home console for them wouldn't work for them. And that they learned from last gen that supporting two consoles (home and portable) would be too much for their teams to handle going forward. With the guaranteed appeal to handheld design it was necessary for them to move that direction.

I've seen several other devs praising Xbox Series X as well. Not to mention Digital Foundry, who got an early hands-on with Series X earlier this year. 

Yet every time someone says good thing about PS5 or its SSD we are flooded with "every dev was paid by Sony" right?

People always troll during console wars. The only praise PS5 has got that seems like it might have been bought was the Epic Games/Tim Sweeney praise, considering we later discovered that Sony bought a portion of Epic Games. 

If he doesn't bother to argue for his opinion, I couldn't care less what he thinks.

No, he was talking about the hardware and the porting capability: https://www.gamespot.com/articles/valve-says-wii-u-fits-better-with-its-tech/1100-6320886/

But that comparison was Wii U vs. Wii, not Wii U vs. 360 or PS3.

"Wii U seems to be a lot more powerful than the previous generation," Valve Corporation president and cofounder Gabe Newell told Joystiq. "It sort of fits better into the scalability in terms of graphics performance and CPU performance, so I think it'll be a lot easier for us to fit it into our scalability model... Now it's a lot easier to look at Wii U and have it fit within that framework."

Vice versa.

Well even before we discover that, considering they made the demo for PS5 so it would be strange if they didn't praise Sony for the system as well.

Would be good if Gabe gave his reason, even if it simple "the specs are higher, GP, play anywhere, etc" even though that is probably what he thinks even without saying.