That's okay, Glen. It doesn't change anything. The bandwidth of the length of cable can be measured. If SNR is also a function of frequency, you measure SNR at the bandwidth you have just determined the cable can support.
The mistake some people make, when they claim or imply they have exceeded the Shannon limit, is that they assume interference to be noise. If some or all of the interference is from the main signal itself, e.g. inter-symbol interference (which is caused by the symbols traveling along the cable becoming distorted and then overlapping with adjacent symbols) or just plain echo, this cannot be assumed to be noise. This is signal energy, rather than energy from other sources. So it must be considered to be S in Shannon's equation.
If you look at Shannon's equation, which I repeated above, it's clear that the theoretical capacity in b/s will increase if the S of S/N is computed as (signal + interference resulting from the main signal). N has to be other sources of noise, which would include random white noise as well as interference from unrelated signal sources.
Absolutely. All in a quest to approach that elusive Shannon's limit.
Bert