I think they mean that most people think the withers being higher than the croup = up hill, when in fact it doesnt.
Anyway This is how I learned about 'functional, up hill' balance - An indication of an uphill horse is when the line between the widest point at the base of the horses neck (where spine intersects the shoulder) & the position of the hip joint (located approx 1/2 way between the point of hip & point of buttock), runs uphill, a down hill horse runs down hill & some horses are neither up or down hill & are level in balance.
See attached image below:)
However from the conformation clinics I've been to, being built "up hill" isn't as nearly important as 4 other key structural points. Lumbosacral joint placement, Stifle placement, Quarters shape & Pillar of support up front. These 4 key points allow for utilisation/transfer of power of the quarters, length of stride & ‘sitting' power, either allow for sustainable compression/loading or immediate compression & spring & forehand lift/elevation.
So just remember to take the whole horse & not just a single aspect of it into account :)