I don't think anyone ever properly answered your question so I'll give it a shot.

In this context, "hop" is a technical term related to ways you can exploit pools running certain older payment methods by changing between pools in a way that will increase your income at the expense of other miners on those pools who remain loyal to the pool and stay there all of the time. Eventually some alternate payment methods were developed which were hop-proof in the sense a miner can't enter/leave the pool in a systematic way in order to boost their earnings. The most popular is PPLNS, although to me the most interesting is DGM (by Meni Rosenfeld). (I ran a Terracoin pool for over a year that used DGM.)

So when p2pool says it is Hop-proof, it means you can't abuse the payment method. It doesn't mean people can't or won't still use multiple pools or move around to chase "luck".

If you are interested in more information, read Analysis of Bitcoin Pooled Mining Reward Systems.