Multi-taper Coherence, With Tapers=1
For any estimate of the coherence, the cross spectra and spectra need to be averaged before forming the estimate of the coherence.  If there is no averaging, the squared coherence will always be 1.  You can see this by plugging in a single Fourier coefficient for X and Y: $C_{xy}^2=(\alpha_x\alpha_y)^2/(\alpha_x^2\alpha_y^2)=1$.  If you instead use a sum or average of two Fourier coefficients for X and Y, the answer can be less than one.

In multitaper spectral analysis, this averaging is done by averaging spectral estimates made using the same time series but different taper windows.  If you only use one taper window, there is no averaging being done.