I'll put it one better. Mariko is a custom Tegra chip, T214 vs the X1's T210. It costs a lot to design a new chip like this, and they still haven't used it, even if they have firmware for the device since March. Here is the real hint IMO, a more power efficient display. Why? Shrinking the chip to 16nm would increase the battery life in the current Switch to over 5 hours with no other changes, 12nm could even possibly achieve 7 hours given voltage reductions... So why care about a more power efficient Screen then? Well if you put a more powerful CPU and GPU in the Switch, you are going to need that extra power efficiency if you want to get up to the 3DS' 3.5hr battery life.
Mariko's firmware data revealed that it has a 8GB profile, it also has a 4GB profile, but devkits for the Switch do not have extra RAM, they simply only have a higher storage capacity of 64GB, Mariko, the Foxconn leak, these Switch "Pro" rumors... it's been climbing in evidence for 2 years that this more powerful device is there. Heck, even the original Semi-Accurate rumor had the deal with Nvidia to sell 20 Million 20NM chips that TSMC had Nvidia on the hook for, which is why Tegra X1 chips cost Nintendo ~$55 a piece, which is half the price of the Wii U. Switch sold 20 million units, but originally it was suppose to sell about half of that, they expected to sell around 20 Million by the end of next FY, and now this rumor comes out saying Mid 2019. This was always in the works... I just watched a video on Youtube about how this is because the last few months of sales have been slightly lower than Nintendo expected... Nintendo had Mariko in it's firmware back in MARCH when they announced a striking sales forecast of 20 Million units, they were in the works with this Switch before the last 6 months of sales, so no, it had no bearings on what Nintendo is doing.
Also, the Security update narrative has kind of been shredded by the supply chain's new components IMO, this is about updating the Switch. New 3DS launched 3 years into 3DS' life, a Switch "Pro" launching just before the 3rd year isn't a big difference, especially when compared to PS4 Pro, that launched on it's 3rd year anniversary.