The paper states that the technique doesn't work when there is a particularly large amount of internal network traffic. If you really don't want to be discovered by your cable company, you could set up a box for the sole purpose of sending and receiving internal traffic. Considering that a cable modem doesn't even run at 10Mbps and a typical LAN runs at 100Mbps, the extra traffic shouldn't matter much.
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Consider photographic authentication. http://doi.ieeecomputersociety.org/10.1109/MPRV.2003.1186723
The paper states that the technique doesn't work when there is a particularly large amount of internal network traffic. If you really don't want to be discovered by your cable company, you could set up a box for the sole purpose of sending and receiving internal traffic. Considering that a cable modem doesn't even run at 10Mbps and a typical LAN runs at 100Mbps, the extra traffic shouldn't matter much.