MPU6050 acceleration values not 16-bit?
The ADC for the accelerometer is 16 bit, yes. Also the data being sent out is in 16 bit format.

But what you are confusing is resolution and range.

The accelerometer can be programmed to have a sensitivity of 2g, 4g, 8g or 16g.  With it set to the least sensitive, that is 2g, the full range of -32768 to +32767 would equate to ±2g. 

That's fine when you're measuring G-forces of up to 2g, but when all you are sensing is tilt then there is only 1g - the 1g the Earth exerts on it.

That 1g is half the 2g range, which is -16384 to +16363.  So round that up and give a little extra headroom to your calculations, and you get -17000 to +17000. It gives enough range in your calculations to give the full ±1g range "plus a little bit".