What you describe is probably true, but here's an alternative experiment. Publish a story NOT behind the paywall but which is similar to a story behind the paywall. Medium will probably throttle the story down after a few days (if it promotes it at all), but Google (as far as I understand Google) probably won't. If the story (title, concept, keywords) appeals to a large audience, then the results from readers outside of Medium should remain constant or even increase over time. The enemy of large-scale readership may be the paywall itself, not any sort of Medium promotional algorithm.