I think they were saying "That BBC documentary shows or exposes very shallow thinking skills <of those involved with designing the canal> at this timestamp". not "<of those making the documentary for not correcting the design of the canal>"

That said, I can't quite understand the complaint. It's either take the freshwater out of the lake and into the ocean or take the ocean water into the lake (or invest build the entire length without using natural bodies). The former seems to make a lot more sense and is effectively what's supposed to happen with water eventually anyways.