Does this motor have a gearbox inside?
No it definitely does not have a gearbox. From other pictures I've found it appears that the tacho is implemented as an extra magnetic disk on the back of the motor that is picked up by the tacho coil. This disk has 12 sets of magnetic poles on it, so for every rotation of the motor shaft 12 pulses will be generated.
So when the motor turns at 1000RPM this corresponds to (1000/60)=16.67 rotations/second of the shaft, and multiplying this by the number of poles on the encoder disk (12) this gives you a 200Hz output signal at that speed.
Note it is highly unlikely that a motor that size actually has 12 rotor poles. It will be either 2 or 4.