It's actually accurate to call them spin-offs. However, we need to make clear of the definition of a "spin-off".
A spin-off is a work that is derived from existing works, telling a story in a different aspect but in the same setting. A spin-off can then either choose to either deviate completely from the main storyline or to remain in it. In the case of the KH series, the spin-offs focus on characters other than the main protagonist Sora (except for Re:CoM and DDD), telling their sides of the story while at the same time, expand the plot of the Dark Seeker Saga (up to KHⅢ) without deviating from it. Re:CoM serves as the bridge between the first two mainline games while the others introduce the background of characters (and are also the bridges between KHⅡ and KHⅢ) and eventually, everything converges and culminates in KHⅢ.
So calling them as spin-offs is not wrong. It's just that people have the misconception that "spin-offs" are supposed to have nothing to do with the main narrative and so the spin-offs of the KH series are supposed to be unimportant. Then, they complain that the spin-offs are too closely tied to the mainline games. It just doesn't make sense.