I think the point is that even though it's smaller and less obvious that the other objects, it's still sufficient to "hijack" the recognition for the whole imag

I think the point is that even though it's smaller and less obvious that the other objects, it's still sufficient to "hijack" the recognition for the whole image. If they had a little sticker with a normal picture of a toaster on it it's unlikely that it would've prevented the banana from being recognised, and the image only looks a bit like a toaster, whereas the banana is unambiguously recognisable.