Rather than going by an arbitrary definition of platform that was likely devised by this site to create a more concise filing / labeling system why don't we actually look at the underlying logic...
If the definition of a platform includes accessories that demand exclusive games / versions of games then there are going to be all sorts of ridiculous entries...
Is each labo design a platform, because to play their accompanying mini-game as intended you need to have the kit?
Ok, I'll try to express my point of view.
By my developer logic, I would say that Wii Balance Board and Labo simply change the interraction in your game, not the game itself in the way you create it, technically. You always develop it with the same technical requirements of any game on the same platform. You only change / redefine the shape.
For Sega CD or PSVR, you have different needs from the "main" platform they depend on, you have different requirements. Not only the interraction, but the display of the game is different, you have to think not only the shape but also the heart.
Sorry if it's not very clear ^^"
The RAM expansion pack for the N64 gave the developer different technical requirements to hit, but again I don't think anyone would consider that a different platform...
So on the one hand, you have accessories which change the way you interact - Wii Balance Board, Labo, the bongo drums, Kinnect and indeed the PSVR.
On the other you have ones that change or improve on the capabilities of the hardware - which would include the ram pack, Sega CD, N64DD etc.
In either scenario, I personally think it'd be silly to class them as their own platform, and I feel that trying to justify the decision to do so is just muddying the waters and, intentionally or otherwise, derailing the thread...