Having a REPL and being extensible seem like totally orthogonal concepts to me. You can have a REPL and be hardly extensible. You can lack a REPL and be extremely extensible. If anything, the REPL is going to expose interfaces that enable extensibility; it's those APIs that are important, not the existance of an REPL.
Technically they are orthogonal, but in practice if you have a repl implementation that's well done, it will be used by the developers, and the developers will ensure that the functionality they need to implement the project is accessible through it. The developers can still screw things up, but it's a factor that encourages them to do the right thing through basic dogfooding.
