> At that point, there's probably not much major input you can have into the core architecture or strategy

Sure you can? In this concrete case, Redis is very "flat" — there's the data structure implementations, and there's the commands that use them. 1+N. You could have feedback about the data structure (i.e. whether it's optimal for the use-cases); or about any of the commands (i.e. not just their impls, but also whether they're the best core API surface to lock in long-term, or even whether they're worth including at all.)

Any given feedback would necessitate fairly limited rework to address, as you're either modifying the data structure (and its tests) or a command (and its tests and docs.)