I don't understand why, tho I do not have any kind of expertise here.
I suggest (Haven't read it), this paper proposes to send much denser and broadened signals around one carrier frequency (they use single mode). Due to dispersion they
Start to overlap with one each other. If you put more frequencies, you would have more overlaps and I fail to see how it won't lead to errors.
They all arrive at the broader time window, which again could be mitigated either by error correction, or by extending the time window.