I have been contracting for a company that has an in-house version of this tool. We are able to push about 10 million emails a day, including processing bounces, click, unsubs, etc, with a fairly straightforward set of python scripts. Its not about the custom logic, its about choosing the right basic building blocks.

Based on my experience, I doubt they need their "highly sophisticated" PHP to do this -- plain old normal PHP would probably be fine so long as they choose the right out-of-the-box software to pair it with. (nginx, postfix, certain non-sql databases, and the right mysql setup)