As was pointed out to us by soooooo many overnight, Kirby will of course boot up and play on the GBA SP. It's just that, as the cartridge is housed upside-down, the game won't be playable, because left and right are reversed.
And what's more, we are assured that the SP is "exactly the same, under the bonnet, as the original."
However, we couldn't help but chuckle at the irony of this mistake of ours. Surely Kirby, only ever released in Japan and the US, has never been compatible with any model GBA because when the game is tilted out of direct, floodlight grade light, it's impossible to see!
Zoink!