Zelda wasn't quite niche, but it also wasn't THIS big. BOTW and TOTK took it to the stratosphere, so there's a kernel of truth to what's being said here but it's masked in a bit of an exaggeration.
I mean, we (apparently, correct me if I'm looking at the wrong numbers) went from 8.5 million copies sold of Twilight Princess, even 14 million between both the N64 and 3DS releases of Ocarina of Time collectively, and completely shattered these records with BOTW now clocking in at around 33 million units and TOTK at over 20 million. I don't think Nabbit's choice of words here was the best but I will acknowledge that they said "relatively". Relative to how it's performing now, that statement wouldn't be wrong.
And no, that isn't really equally true of Mario and Pokemon. Mario Odyssey and Mario Wonder, although they performed well, haven't reached the heights of previous entries like Super Mario Bros and the first two New Super Mario Bros titles. The Switch Pokemon games still haven't beat the originals on Game Boy. Both of these series moved units in the tens of millions on a consistent basis. But BOTW and TOTK has left every other Zelda game in the dust.
It's true that just about every series does better on the Switch, but let's not downplay how much of a glow-up this generation has been for Zelda specifically. "Niche" may not be the right word but putting that aside, the sentiment holds to a degree. How much of that hinged on BOTW being a launch title I'm not sure, but I'm sure that it helped at least a little bit.
To wrap it back around to Smash, I feel like seeing this much unprecedented growth for an already popular series should reignite some optimism toward new Zelda content in the next game. It's still hard to figure out exactly how that will pan out, but it really feels like the right time if any.