Depth Cameras can help a VIO system in a couple of ways. Accurate measurement of ground truth & metric scale and edge tracking for low features scenes are the biggest benefits. They are very power hungry, so it only makes sense to run them at a very low frame rate and use VIO in between frames. They also don’t work outdoors as the background Infrared scatter from sunlight washes out the IR from the depth camera. They also have their range dependent on their power consumption, which means on a phone, very short range (a few meters). They are also expensive in terms of BOM cost, so OEMs will avoid them for high volume phones.
Measuring distance using the IMU is called Dead Reckoning. It’s basically a guess, but the guess is made accurate by modelling how the IMU behaves, finding all the ways it accumulates errors then writing filters to mitigate those errors. Imagine if you were asked to take a step then guess how far you stepped in inches. A single step & guess would have a high margin of error. If you repeatedly took thousands of steps, measured each one and learned to allow for which foot you stepped with, the floor coverings, the shoes you were wearing, how fast you moved, how tired you were etc etc then your guess would eventually become very accurate. This is bascially what happens with IMU calibration & modelling.