That is exactly the point there. When we are talking about protest voting currently, we are talking about these small players, but perhaps there would be groups within our existing parties that would splinter off to create more specialized, let's say, interest parties, more directly related to their own interests. Those would be smaller parties, but they would be more likely to get the support.
The idea is that parties have the incentive to enter the electoral arena if they are more likely to get elected. Voters have the incentive to vote for those parties when they are more likely to get the seats. You tend to see a greater spread of interest being represented in seat-earning parties in more proportional systems.