"just for replication rate to vary depending on some property of the thing replicating"

That's a heritable trait, not selection pressure. Remember, we're talking about an infinite population size here, but if the pop. size was capped then the mechanism that decided who got to replicate and who didn't would be the selection pressure.

Can you ELI5 why you think the varying replication rate is, by itself, a selection pressure? Because your point's obviously going over my head, and your last comment did nothing to communicate it more clearly.