A PFC in CRM mode uses a variable frequency, depending on the input voltage: check eq. (1) and (5) of your paper. When input voltage changes, duty cycle changes (eq. 1) and switching frequency depends on duty cycle (eq. 5).
Moreover there is a second phenomenon not considered in that paper: the triangular waveform at variable fs (frequency modulation) has also an amplitude variation according to the input voltage (amplitude modulation).
So, when the input voltage changes, it modifies the triangular current waveform both in frequency and amplitude, 100 times per seconds, and this is a modulation, both FM and AM.
You can see the frequency modulation as the carrier frequency swinging up and down in the spectrum domain (naive interpretation) or as a fixed frequency carrier with many pairs of sidebands around it, each spaced 100Hz from the previous one (frequency modulation is mathematically complicated, a lot of Bessel's functions!)
If the frequency modulation index isn't too high you will have a limited number of sideband pairs, or you can imagine that the carrier frequency isn't walking up and down the spectrum too much and it will stay within the bandwidth of your EMI receiver, tipically 9 kHz wide for conducted emissions.
I don't think that the FM spectrum will stay within 9kHz, but I didn't put numbers in the equation and I didn't read all the paper.
In conclusion: you can see the current spectrum as a series of lines at an average switching frequency fs (so you have fs, 2fs, 3fs....) and around each of these carries there are sidebands at 100Hz above and belove, 200Hz, 300Hz... a very complicated spectrum!
My impression is that the spectrum around each carrier is wider than 9kHz, so the problem is more complicated, but you get some noise mitigation because your receiver doesn't measure the total power around each carrier.
Complicated stuff, not easy to explain!