Hm. I don’t recall that being mentioned in the documentary. I mean - it wouldn’t make sense. Injection moulding dies are really costly and it would limit what you could make. Do you remember when this was said? What era ILM?
Even then, California’s ILM didn’t make the Empire props. That was done by a team at EMI in England.
Perhaps this is a reference to the use of vacuforming in the original Star Wars film? The stormtrooper armour was HDPE and ABS vacuum-formed plastic, done by an outside firm. They also got a machine at EMI for vacuforming interior wall panels for the Death Star, blockade runner interior walls, etc.