I mean, I'm guessing it's just saying the bit rate depends on the signal frequency, but it's still funny to see canceling units.
Related: a "spacing" measured in GHz.
Again, it makes sense, but it struck me that someone could write a total spoof paper with nonsense units and I wouldn't be able to tell the difference!
The fact that the time-unit cancels out just shows that spectral efficiency is independent of any sense of time one might have.
Measuring "spacing" in GHz makes sense if you consider the way heterodyne mixing shifts the signal around, without affecting the spacing of sub-carriers.
Frequency spacing makes sense as namibj points out. Most long-distance telecom links operate in the optical C-band, which is roughly 5 THz wide. (A wavelength of 1525nm has an optical frequency of 196.5 THz, and a wavelength of 1565nm has an optical frequency of 191.5 THz). You can select optical frequencies to modulate within this optical bandwidth. Given a certain modulation rate (>>GHz), separating the channels in units of 1 GHz is reasonable.
[0] https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...
One can keep going to higher order modulations to improve the spectral efficiency but the SNR required increases exponentially with constellation order.