Assuming that the average node defaults to 8 connections, then with only 10% of nodes running full RBF then each node has a 57.0% chance of connecting to at least one full RBF node. If 20% of nodes run full RBF, then that becomes a 83.2% chance. It certainly doesn't take a majority to run full RBF for miners to be relatively certain they will see any replacement transactions.
To extend this further, anyone can adjust the number of connections for their benefit. By default, if you allow incoming connections you can have up to 125 connections AFAIK (117 incoming, 8 outgoing). If a miner wants to be sure he'll listen to the double-spending, he can establish a connection with every node of the network (as long as it's bandwidth-wise possible of course).
I think projects like bitnodes.io do make use of this kind of network search tricks.