Complex sentence: The old appointment is assumed to be why Staff Sergeants are referred to and addressed as "Sergeant Major" in British cavalry regiments and can also command a cavalry troop, in the same way as the old WOIII TSMs.
Split into simpler sentences: The old appointment is presumably why Staff Sergeants are referred to and addressed as "Sergeant Major" in British cavalry regiments. A Staff Sergeant can command a cavalry troop, in the same way as the old WOIII TSMs.