We will discover if this is true in the Mario kart World Direct that will happen on 17th Aprii.

Source

I mean, if that's true, it would somewhat justify the price, but only very somewhat.

I thought I read that only playing on the switch itself would offer 120, but not via output to a TV. Is that true?

An ambitious target if true; even on PS5 and Xbox Series X not a lot of games manage a stable 120fps.
Prime 4 does, but that's a game built for Switch 1.

In TV mode the console will support 720p/120hz, 1080p/120hz, 1440p/120hz and 4k/60hz

Mario Kart at 120 FPS in a 200cc circuit will be like:

Metroid Prime 4 with 120fps as well.. Would be crazy if Nintendo goes from games struggling to run 30fps on the Switch 1, to all of sudden Nintendo is pumping out a bunch of 120fps games.

Most first party Switch games didn't struggle to hit 30fps; many were a solid 60 (Splatoon 2/3, Mario Kart, Smash, ARMS, Metroid, etc) and those that targeted 30 generally got there the vast majority of the time.

According to digital foundry the game is 1440p60, so yeah it could have a 1080p or less 120fps mode.

Personally I'll stick to the 60fps modes.

It would be very odd if S2 became the home of high frame rates haha.

It also makes me think though that perhaps MKW started development on S1 as they're potentially leaving a lot of processing power on the table...

That wouldn't be surprising as MK8 came out forever ago.

From the footage I've seen there's a lot of flat areas going on, I'd have preferred a lot more undulation throughout, but I'll reserve judgement until I play it myself.

These are admittedly half-baked opinions atm.

EDIT : if zippy is correct, why would Nintendo settle on 1440p / 60 as opposed to using DLSS to upscale from 1080 to 4K?

Is DLSS gonna be like AA where Nintendo just doesn't bother with it?