The word "difficult" may be interpreted subjectively, i.e. what is difficult for one may be easy for another and what is easy for the first may be difficult for the second, but in any case I have not compared the difficulty of the control loop of a frequency comb with the difficulty of another component of a frequency comb, but only with the difficulty of the control loop of a PLL, which is the device replaced by a frequency comb.

There is no doubt that the control loop of a frequency comb is much more difficult to design than the control loop of a PLL or of any other simple regulator with a single degree of freedom.

Perhaps you have designed yourself the control loop for a frequency comb and it is indeed "very simple", but I have seen tons of books and of research papers about optical comb frequencies, which describe a lot of details about other parts of the optical frequency combs, but none of them ventures to present the details of how the control loop actually works.

While I have designed PLLs, I did not have the opportunity to design a frequency comb, so perhaps the additional difficulty is not great, as you claim, but I suspect that this is not true, because if it were easy to describe that in a few words such a description would have existed in one of the many publications about frequency combs.

In any case, when you claim that a text is "confidently incorrect", you should better point to the exact affirmation that is incorrect.

When you just do not agree with some subjective assessment, like whether something is easy or difficult, using the word "incorrect" is itself incorrect.

In this particular case you cannot even disagree with my use of the word "difficult", because if you claim that designing the double control loop of a frequency comb is not more difficult than the single control loop of a PLL, that claim is certainly incorrect.