They could use the X2 for the slimmer Switch, but it doesn't look like it from the leaked Switch OS hack, there's something called a Tegra Mariko chip which has a model number T214 (Tegra X1 is T210), the Tegra X2 has a completely different model number.
1.5 TFLOPS at FP16 is only half that at actual FP32, which is what most devs use, so 750 GFLOPS at 15 watts is alright, but Nvidia can likely do better than that for 2020. 15 watts may be a bit too hot as well, the current Switch runs at 11 watts docked.
I don't think Nintendo necessarily wanted to go with an off the shelf part, it may have been a decision based on time/cost, but it's also had some headaches for them like hackers being able to easily exploit the X1.
The new part number doesn't necessarily mean a die shrink or even a major chamge. It could be the old X1 chip with the needed hardware changes to improve its security and fight against the homebrew and emulators going around.
I see you catched my trick with the X2 flops, good job . In any case, an X2 should be more than enough to run current Switch games at 1080p@60fps without troubles. Forget about 1440 or 4K, just because the competence has done that, it doesn't mean that Nintendo should follow their path. And you wouldn't need to run the X2 at full speeds to achieve those results, saving power.
Another thing to consider is that using the X2 for both devices would be better for Nintendo as they could get a better deal for the chips due to the higher volume. The extra RAM will prevent users from buying the Lite model and "unlocking" it to the Pro one.
Remember that Nintendo has to eventually launch a Switch 2, and if they go full power on a Switch Pro, what they'll use for it?
I think Nintendo will do away with the "hard system relaunches". It's really not good business to be honest to reset back to 0 every 5-6 years and flush basically all your hard work down the toilet. Especially now that they don't have a secondary hardware platform to bank on, what would have happened to Nintendo during the GameCube era if GBA was not around? What would've happened to Nintendo during the Wii to Wii U transition if 3DS wasn't there? It's too risky to just bet everything on Switch 2, audiences are fickle and can be gone at the snap of a finger, I think Nintendo will change this whole system to protect themselves.
I think they will instead adopt a more Apple like model of new Switch hardware every 3 years and 2-tier hardware approach. Meaning one Switch model that occupies a cheaper price point ($200-$250) and one that occupies a more profitable higher price point ($350) and as years pass, the higher end model becomes the cheaper one and a new higher end model takes the $350 slot. Sony/MS have already discovered this, and Nintendo is already doing this too in a way with the 3DS and Switch being both sold. I think they will want to keep that setup.
It's simply smarter business. There won't be a "Switch 2" so much as there is a "Switch Model 1, Switch Model 2, Switch Model 3, Switch Model 4" etc. etc. etc. and Nintendo won't make as big of a fuss about every hardware iteration, it'll just be "oh another Switch model" all under one ecosystem, branding, account system, etc. That's my prediction anyway, just like Switch has completely change the rules for the handheld/console delineation in hardware terms, Switch will also break the traditional Nintendo hardware lifecycle formula and embrace a more Apple like one.