More confirmation:

“It was no more difficult to move the stone up the ramp than it had been to move it on level ground”

I’m sure most of us reading that would have found it hard to credit, especially as the slope was 1 in 8, but I can confirm that, on my small scale basis, I found you were exactly right.
Having thought about why it happens, I realize that I shouldn’t have been surprised. When you’re rowing 1 in 8 uphill, you could maintain precisely the same lever action as if you were on the level, except that the maximum forward travel of the stone will be restricted by 12.5% due to the front of it hitting the slope. Thus, you might get the impression that it’s actually marginally less effort per (shorter) stroke when going uphill. To counter this, all that is required is that everyone levers the stone a bit higher on each stroke, thus maintaining an unchanged forward travel per stroke. It’s clear that if you’ve got things arranged so that at each stroke everyone is levering the stone fairly comfortably, then they are hardly going to notice the extra effort in levering it a little higher during their stroke. Thus, they’ll be able to row up a hill at much the same speed as on the level.
I’m not sure what the limits of slope are – I guess you’d eventually be forced into limiting the forward travel per stroke, so it would be slower, and there would also come a point where the levers wouldn’t grip or the brake wouldn’t work, but I’m convinced that the limiting factor wouldn’t be exhaustion or numbers of people. If 20 people could move a stone along the flat, 20 people could move it up a steep hill. That’s more than can be said for all the other methods.