With all due respect, I believe you are overthinking this. It's likely that a pure level sensor only would be more complicated and expensive to implement while being less performant at the same time. .
Bosch already makes these inhouse for many applications, so undoubtedly they are using their own sensor that they already have programming for. A 6 axis IMU is basically a gyroscope and an accelerometer rolled into one. It's what anything that moves and has a circuit board uses to tell how it is moving through the environment, including inclination.
FWIW, Yamaha implemented a simpler version of this in their motors a while back. I haven't heard many folks singing the praises of the auto mode, but maybe it's gotten better. I suspect you would find the delay before it reacts unacceptable and at least in the older motors it just changed to the next higher assist level, so there was no granularity in how it provided assist.
