Reading acceleration on the Bangle.js

I just assumed the accelerometer of the BangleJS would be of the same type like those built into smartphones.

It should be identical. In fact if you try https://intel.github.io/generic-sensor-demos/sensor-info/build/bundled/ on your phone (choose + then accelerometer) you will see sensor values that include gravity too.

It's measuring gravity because that is an acceleration it is experiencing - if you were to throw it in the air then while it was in freefall you should see a proper 0 accelaration value.

If you want to try and remove the effect of gravity then for the car acceleration case I'd just take a reading when the user presses the 'start measuring' button and then subtract it from what you get next time.

However, actually getting a proper 0 value when the watch is on your arm is harder than it seems - realistically you need a gyro as well (which Bangle.js doesn't have) so the Bangle can keep track of what angle it is in in 3D space and then subtract the effect of gravity.