I don't think "Dynamic Thermal Guard" is a reference to any SD thermal management spec. It seems to be something Samsung advertise for many/all of their NVMe SSDs, so likely got carried over to their microSD Express controller. I feel like it's custom just by virtue of being something Samsung wouldn't have bothered making without a customer lined up.
It's quite possible that this is for Nintendo, but two things give me a bit of a pause. Firstly, if Switch 2 is launching in March next year, then it would be odd for Samsung to start shipping "custom" microSD Express cards this year, leaving them on the market for months before the only device which supports them launches. It's also a little strange that they've only announced a single capacity. If Switch 2 were launching with a new storage standard, I'd expect there to be enough demand to have a range of capacities available at launch. Having only a single capacity on the market makes me think they might be intended for a product with lower sales expectations, like a drone.
Also, I fully expect T239 to support PCIe 4. Nvidia have supported it in their SoCs since Xavier (Orin Nano is limited to PCIe 3 for binning/product segmentation reasons, but it's the same silicon as Orin), and they were taping out products with PCIe 5 support (Grace) alongside T239, so there'd be very little reason for them to revert back to the PCIe 3 controller they last used on Parker. Particularly so when there are areas, like external storage, where Nintendo could benefit from PCIe 4 speeds.