Key travel time is itself a form of latency, distinct from throughput because it doesn’t count the time you take to move your fingers between keys, because you can start pressing a one key while the previous key is on the way down (as long as it’s not the same key), etc. But even aside from that, if key travel time is the biggest contributor to a keyboard’s overall input latency, then that’s what it is. There’s no real way to avoid it (except maybe pressing the keys harder?), so it doesn’t make sense to exclude it just because it’s mechanical rather than electronic.