I agree this is a weakness. I do agree that strolling does not need to be absolutely level and I think it is hard to give precise number, it is a combination of smoothness and incline. Very smooth surface could support a bigger incline then a slightly less smooth surface. In general I would say incline should be under 10 % or so.
T0 would probably better be defined as “Terrain level or (very) slightly inclined; no risk of falling or tripping over minor obstacles.” Strolling should not be steep, but “steep” is indeed vague.