The 20X0 series launched in 2018. The switch 2 uses the same architecture.
As others have pointed out, the T239 is using the Ampere architecture, which launched at the end of 2020. But more to the point, the T239 is based on the Jetson Orin NX, which launched in early 2023. So it will effectively be 2 years old at launch. The Steam Deck uses RDNA 2, also launched in 2020, and while the ROG Ally uses RDNA 3, RDNA 3 is like 5% faster at the same clocks.
We should note that the T239 has 1536 ALUs, which is double the ROG Ally's 768. Everything else is not doubled, but on the same process and TDP, we would expect it to be a good 20%-30% faster than the ROG Ally. So Nintendo's supposedly super unambitious design has the potential to beat the ultimate PC enthusiast handheld at a given TDP. Obviously this depends on Nintendo using a modern process.
(You will probably say that Nintendo would never use a modern process, but people have done the analysis and found that the large size of the T239 means it will hit the voltage floor on older processes at mobile TDPs, so in that case Nintendo would have been better off from a cost and performance perspective shipping a smaller GPU).