sorry, you're wrong, all the Lego gyros can calculate yaw angles, just by Gyro values.
Perhaps they are not stabilized against accelerometers, but they still can.
Nonetheless, by evaluating addionally accelerometers one can retrieve at least the horizontal gyro rotation fraction (== yaw) in case the sensor has been tilt, just like for pitch and roll.
The yaw starting angle can be initiaized at program start by an arbitrary value, like Lego does it, too; the follow-up yaw angles are then relative to this one, as already stated.
So how to retrieve the yaw angle by a MPU6050 by which lib, and even when not Kalman-filtered?
(Anyway, the raw or sole gyro + acc values may be Kalman-filterd though)