Bakster
If he did I cant find it
Neither can I--and I looked a bit deeper about for other video on this.
The choice of concrete is one I'd love to ask the creator about. Most folk intending to RC at such size simply use sealed motorcycle batteries to help ballast down.
That, and water balast is cheap and free, and reversible. And, he mentioned having bilge pumps aboard as is.
Few things as nagging as "What if?"