True in theory…
However, in the case of the Rossini for example, it uses a discrete part for its Ethernet port Magnetics; Pulse Electronics H5062FN <— Click on that link to have a look at its specifications; signifiant noise attenuation well into 200MHz!!
I’ll leave aside subjective discussions of the audibility of those kinds of frequencies ever impacting the DAC. I will just say though, IMHO, the dCS Engineering folks know what they’re doing 