There is a comment from on the page you linked saying 'less than Splatoon'. I agree, it's not a consensus opinion, but there are clearly some people who thought MGSV would debut lower than 500k.
I'm pretty sure that wasn't a 3DS game when that original trailer was made. The game was announced with no platform back then. It then disappeared for ages and re-emerged for 3DS looking totally different.
Regarding those 1280 x 768 screenshots of Monster Hunter Stories, it's highly likely to be the 3DS game. Capcom have used high resolution renders of their MT Framework games before, and just like in those instances, we're seeing the exact same assets (low resolution textures, etc.) that'd look great on 3DS now rendered at a higher resolution, exposing their blurry origins.
1280 x 768 is a 5:3 aspect ratio, the same as the 3DS itself (400 x 240), and another MT Framework game, Dai Gyakuten Saiban, was also rendered at 1280 x 768 in trailers. See: https://youtu.be/X7fl2xDedO8?t=122
Crumbs, someone mentioned these 1280 x 768 screenshots of MH Stories, and I thought they were from a 2016 thread. My reply has lost all its context now...