There are Game Boy Color exclusives like Pokemon Crystal that could not be played on other Game Boy hardware iterations available at the time. So are Pokemon Gold and Silver, released in 1999 and marketed as Game Boy Color games, considered exclusive to that platform, even though other Game Boy models capable of playing the games had been on shelves for over a decade?
It's a similar situation with Xenoblade Chronicles 3D on the New 3DS. So that game is (handheld) exculsive to the New 3DS because it can only be played on that specific model. However the new 3DS is a new model of an already existing platform, not an entirely new platform, so you could say it's exculsive to the 3DS family and still be correct.
The GBC is basically an upgraded Gameboy, although different enough that is has to be distinct but not different enough like the GBA to classify itself as an entirely new platform. At least that's how I think of it anyways, some people may disagree and suggest the GBC is it's own platform different from the Gameboy.
Since Gold and Silver are playable on both the GBC and GB then it can't be an exculsive of either, but it is exculsive to the 8bit Gameboy systems.