Well these are the colors they define:

    --primary-color: hsl(var(--hue), 50%, 90%);
    --pre-primary-color: hsl(var(--hue), 50%, 95%);
    --secondary-color: hsl(var(--hue), 50%, 10%);
    --tertiary-color: hsl(var(--tertiary-hue), 80%, 20%);
    --accent-color: hsl(var(--accent-hue), 80%, 20%);

As I see it that's either 3 colors or 5 colors but not 4

White and grey are shades, rather than typical colours. Any hue with 0 saturation will be a shade between white and black.

(Yes, probably there are arguments against this, but in terms of colour theory for dummies.)

So is a change of hue required in order to say there's been a change of color? I'm not offering saturation in this question, since "oh that's just a less saturated red" seems similar enough to the notion of "oh that's just a lighter red" that the two ought to be in the same "not color" boat...

Assuming the hue and saturation components stay the same, changes in lightness would result in various shades of the same “color”. I think. Maybe. The problem is that in real life changes in lightness result in changes in saturation as well, so things get wonky.

I think that’s more of a question of semantics than colour theory, but maybe? Is pink just light red? Feels like the answer should be no, and that undermines my basic premise above, so I find myself doing mental gymnastics trying to answer your question, which more often than not is a sure fire way to tell that I’m wrong.