He is saying a video game running at 60 Hz has approx 16 ms per frame. 550 GB/s * 16 ms ~ 9 GB. So if you are running full bandwidth for an entire frame you can access 9 GB of RAM.
What the responder was getting at is that despite being able to access only 9GB per frame, it may still be useful to keep more than 9GB of data in there for other purposes, say if you have data that isn't being read/written every frame but is still used for rendering. So it doesn't necessarily follow that memory beyond that which can be addressed per frame is useless.
