The black hole engines he's referring to glow very hot because they're very small, meaning that the surface of the hole is large compared with its volume. This

The black hole engines he's referring to glow very hot because they're very small, meaning that the surface of the hole is large compared with its volume. This allows for very bright hawking radiation if I understand correctly. If you stop feeding them they disappear within hours, years, or decades depending on how big they are.