I think there is some cross-confusing of crabs. Sally lightfoots like we have in our tanks are surely not the ones in such a documentary. A tropical, reef area crab wouldnt be found by penguins. There are thousands of crab species, that are flattened, and probably even bear the same silly common name of "Sally Lightfoot". But, surely not the smae species. Our aquarium ones are Percnon gibbesi</em>, and only get a few inches.
In fact, look here:
http://megan-jungwi.suite101.com/predators-and-prey-of-the-galapagos-penguin-a199338">http://megan-jungwi.suite101.com/predators-and-prey-of-the-galapagos-penguin-a199338</a>
It says "sally lightfoot crabs" are predators of penguin chicks, but the scientific name is [I]Graspus graspus</em>, a totally different crab.