The leak is from Famiboards who have the best discussion on the tech of Switch 2, their users found shipment testing data of actual Switch 2 hardware which basically confirms Tegra T239, 12GB LPDDR5X RAM, 256GB UFS 3.1 storage, but also the new dock has a fan inside of it, it's publicly available data but you would have to know what you are looking for and understand what the different product codes mean.
Looking at that, even regardless of what the Tegra GPU is clocked it (it's irrelevant because it costs the same either way, 1536 CUDA cores is 1536 CUDA cores no matter what, you don't pay for 1/4 of an apple if you take two bites and eat no more, you pay for the whole apple, the grocery store doesn't care if you eat all of it or not), the screen size, the RAM, the internal storage ...
I don't see $399.99 at all. This is $449.99 most likely. As I've said there are $1000 premium phone and tablets with LPDDR5X RAM and that much storage, I doubt Nintendo is going to gift that away for only $50 more than a Switch OLED. The iPad Pro OLED just released like yesterday and that has only 8GB LPDDR5X RAM and that's $1000+.