Nintendo will never confirm mass production, since when have they ever done that? Maybe Sony operates differently, but you'll never get that out of Nintendo.
We know from customs/shipping records that the Vietnam based factory that mass produces the Switch 1 are receiving Switch 2 components and we even know which components those are. As of June/July last year it was in low quantities, but there's been a monstrous surge in late July and August and they are now monthly receiving shipments tens of thousands (even more) of Switch 2 components on a monthly basis. Things like the Tegra T239 chip and Switch 2's specific LCD display, along with other less sexy parts like inventory for screws for the Switch 2 went from a small amount to now being stocked in the millions there.
That isn't speculation or opinion, that's a fact. That is the customs/shipping record.
I would say that points pretty hard towards production about to begin, otherwise why would you be sending large numbers of to the manufacturing point.
There's also seperate data on Nintendo's financial sheet that shows they are having the largest work in progress component surge of the last 6 years, these spikes always correspond to new hardware being released in the past (this number spiked 6 months before the Switch 1 launched, 6 months before the Switch Lite/Mariko launched, and 6 months prior to Switch OLED launching). Guess what happened in September 2024? It is spiking through the roof.