The short answer, I guess is "easily." The long answer is slightly more nuanced.
Basically, it's like
@ILikeFeet said - the Steam Deck is a mini PC, and that means that it has to run PC CPUs, instead of Nintendo's mobile CPUs. If all you did was replace the CPUs, you'd save something like 25% power right away.
Steam Deck is built with last gen tech. Its using a TSMC 7nm process (which is very nice, don't get me wrong) but modern GPUs have recently moved to 5nm, which offers something like 30% power savings. And modern GPUs have other power saving tech, which we know Nvidia has at least experimented with putting into the new Nintendo chip.
So yes, Nintendo can build a chip that is more powerful than Steam Deck and draws less power. I think most of us expect a more complicated situation. There are places where Nintendo could choose to be
more powerful than Steam Deck (the GPU), places where it could choose to be
less powerful (the CPU), and places where it might actually be
both - more powerful in Docked mode, less powerful in handheld mode (Memory clock).
There are other places too that Nintendo might be able to save power or save costs over Steam Deck. Steam Deck has a multiple track pads, reassignable grip buttons, and touch sensitive sticks. Steam Deck also uses a custom LCD screen, which is very bright, and is a bit of a power hog. Steam Deck has 16GB of memory, but spends a lot of it on the OS where Nintendo doesn't.