Scott, that’s a good observation – I hadn’t noticed that part of the specification. However, in practise it would not be a great problem and certainly doesn’t render the unit unsuitable.
My Leica disto D3 only measures up to +-50 degrees inclination. However, while verticals (90 deg) are quite common, I hardly ever find myself measuring very steep slopes (which surprises me, but it is so).
On those rare occasions I simply estimate: I interpolate between 50 degrees, which I can reach on the instrument, and vertical, which I can judge quite well. The method is to point the laser spot up (or down) at the max (50 deg), then pass through the target point and get to a vertical position of the disto, and then estimate how far the target pass-through was along that tilting motion. Sounds complicated, but actually easy to do.