I place very little faith into MLID's "sources", but I wouldn't be too surprised if 1.3GHz (and therefore 4 TFLOPS) was an internal target for Nvidia, even if they didn't expect Nintendo to clock it that high. Even if Nintendo had settled on, say 1.1GHz as a target clock in docked mode during the design process, the actual clock speed could end up lower or higher than that based on the performance of the silicon, other hardware design changes, etc., so there would want to be some wiggle room in achievable clocks to accommodate that. A 1.3GHz clock is probably a reasonable upper limit of what Nintendo may choose, and also a round number in both clock speed and Gflops, so it's a plausible target for Nvidia. Besides, every other Ampere GPU can clock well past 1.3GHz, so it's really quite a modest target for the architecture.
On the LPDDR5X side, I could see it happening for two reasons outside of performance requirements. The first is simply that Nvidia was already working on an in-house LDDDR5X controller (for Grace) with a similar tape-out timescale, and it may have been the case that using the updated controller would have a near-zero impact on cost and timelines, so it was basically the default option.
The second is availability of LPDDR5/X RAM down the line. Back when Nintendo and Nvidia started supporting LPDDR4X with Mariko, I assumed it was purely for the sake of the power efficiency benefit. This may have been partly the case, but in the years after release, LPDDR4 has all but disappeared from the market, with LPDDR4X almost completely replacing it. Being able to use a widely available (and therefore cheap) form of RAM for the rest of the console's life was likely a major reason behind switching from LPDDR4 to 4X.
With Switch 2, it seems pretty unlikely that there will be a Mariko-style updated SoC at any point in the console's lifespan, which means they'll need to design a system which they can still buy parts for perhaps as much as 8 years later. Nintendo (and, I'm sure Nvidia) have surely spent a lot of time talking with RAM manufacturers about the production lifespan of the parts they're interested in, and if it's expected that LPDDR5X will almost completely replace 5 in the same way 4X replaced 4, then there would be a very strong incentive for Nintendo to have an SoC with LPDDR5X support even if they had no intention of using the higher speeds.
