Being tracked only really solves the traction part of the equation so they aren't necessarily any better at navigating slopes safely than a wheeled mower.
I thought the same thing about tracks: only solves the issue of traction/ground pressure, but for whatever reason they're supposed to be better on slopes.
My (wheeled) mower sometimes slips or gets stuck on a particular area on my property because I often don't have enough traction to mow up & down the slope if the ground is not extremely dry: grass is enough to cause slip. And sidehilling causes the upper wheels to lose traction. Perhaps the increased "bite" of the treads compensates for this? Dunno.