I don't know what kind of systems you worked on in your career, but even simpler systems with smaller userbases than twitter are quite complex if you are new to it.
Twitter serves their service to the entire world, with multiple layers of systems working in conjunction in order to make things work smoothly. A new engineer that has not been working on it for no more than a couple months would likely be unaware of how the different systems communicate and interact. A change like this will have have a lot of unintended consequences, and not having a senior engineer with lots of context leading the change will undoubtedly cause these kinds of issues.
Twitter serves their service to the entire world, with multiple layers of systems working in conjunction in order to make things work smoothly. A new engineer that has not been working on it for no more than a couple months would likely be unaware of how the different systems communicate and interact. A change like this will have have a lot of unintended consequences, and not having a senior engineer with lots of context leading the change will undoubtedly cause these kinds of issues.