So a couple of things:
1) When the switch first came out in 2017, the hardware was roughly on par/comparable with the ps3 released back in 2007. That's a 10 year gap. Unless there's been some sort of major change recently in the industry, I see no reason to believe why the next gen 2024 switch wouldn't be able to equal the performance of the 2014 ps4 at the very least and very likely to exceed that level with features like dlss.
2) If we take a look at history, Arm iGPU's have always been slightly ahead of x86 iGPU's, at least in terms of GFLOP/TFLOP measurement. This has been the case for last decade or so. Performance has however been a mixed bag and not directly comparable on arm side due to subpar driver support. This does not apply to Nvidia tho, as they're the only arm licensee vendor that provide excellent gpu driver for their soc's which is truly directly comparable to their desktop geforce line.
3) Again looking looking at history, Arm's SoC's are just massively more efficient than x86 ones in general (just take a look at how efficient M-silicon macbooks are compared to windows laptop alternatives). I think you're severely underestimating this point. If the cpu's are able to power down to a level that takes 2-3x less watts this leaves significantly more headroom for the igpu, where every watt counts. Even if Nvidia/Nintendo are to be extremely conservative and use some outdated process node like samsung 8nm for their soc, it'll still have order of magnitudes better battery life than the deck just based on this alone.
4) The steam deck already struggles to deliver ps4 like performance in games. Not because the hardware isn't comparable but more due to lacking the same level of developer optimization that consoles get.
5) TL;DR -- I feel the author of this article wrote sensationalist headline here. It doesn't need to be a killer (altho it's shaping very likely to be one), this is Nintendo we're talking about. They're like apple, anything with their badge on it sells millions. Heck, they could pretty much re-release the Wii U (one of their worst selling consoles) and it'd still sell another 10 million or so and outsell the deck.
These aren't even comparable devices, haven't even gotten into how they're marketed differently (online presence only limited region availability vs physical store presence worldwide availability). Or how many are not too happy about the physical size of deck, the weight, and fan noise. I could go on about shader cache eating storage vs having none dealing with stutter and many other issues but I think I'll stop here.