"Of course, the device will have an IMU of some kind, but Lenovo wouldn’t even cop to its existence, let alone offer any details on it. (The only way it wouldn’t have one is if Lenovo used sensor fusion, which seems less than ideal on such a pricey item.) "
You have a fundamental misunderstanding on the purpose of Sensor Fusion. ALL current viable VR HMDs utilise Sensor Fusion between an IMU and a supplementary absolute positioning system (most use nIR optical, but there are RF and magnetic systems available). The LACK of Sensor Fusion would be an indication of a substandard system. This is because an IMU alone is not sufficient for accurate position tracking due to extreme drift from error accumulation (a limitation of Dead Reckoning navigation), and a commodity optical/RF/magnetic system alone is not sufficiently performant (low sample latency, high sample rate, high instantaneous sample precision).
Quite simply: Sensor Fusion is the method of combining data from multiple sources to produce one output. If you are not using Sensor Fusion, then you are not using multiple sensors. Multiple sensors are a fundamental necessity for VR tracking today, ergo any VR HMD will either be using Sensor Fusion, or be a failure.