Frequency combs are optical synthesizers. This means that the frequency of the nth comb mode is exactly n times a radio frequency (+ another radio frequency), where n is a (very large) integer. The speed of light does not matter at all. This is very important since the speed of light (and hence the wavelength) depends on environmental factors such as air pressure and humidity. One way to determine the speed of light (in air) would indeed be to measure the wavelength of a laser whose frequency is calibrated using a frequency comb.
The speed of light in vacuum cannot be measured in SI units since it is the fundamental constants that defines the unit "meter".
The speed of light in vacuum cannot be measured in SI units since it is the fundamental constants that defines the unit "meter".