diff is a numeric function. categorical is not numeric. Ordinal categoricals have a mathematical ordering, but they don't have a "distance", so diff makes no sense for them.
But diff(x) is just x(2:end) - x(1:end-1). Again, categorical isn't numeric, but remember, the "trick" is not just diff(x), it's diff(x) > 0, so what you want is
d = c(2:end) ~= c(1:end-1)
At that point, you are exactly where you'd be if you had started with a numeric vector. Your runs (including singleton runs) begin at find(d) and the run lengths are diff(find(d)).