Having the clarity over what's important allows teams to perform rather than endlessly debate over tradeoffs.
It still feels like a shaky premise to me because had USB-C not been mandated, they'd never have prioritized it. I think they waited until the last minute simply because they could.
Again, all speculation but the point is how a defined end point allowed teams to receive clarity over tradeoffs and move quickly. It's very typical of how Apple works (which you can see in Vision 1.0 where certain things were wildly overspecified and other were wildly under for that reason). The exact details may be wrong but the logic of how they got to this place feels right.