It's not clear from the article, but my guess is (as others noted) they'd have some level of confidence from testing against the other inhabitants of the island

It's not clear from the article, but my guess is (as others noted) they'd have some level of confidence from testing against the other inhabitants of the island. They could engineer the baits to look, smell, and taste especially unpalatable to the birds and iguanas there, or ship the baits in wrappings that rats could & would be able to penetrate, say a thick cellulose or cardboard wrap containing a single serve. Typically the problem with baiting in this way is what happens when those animals die, and other animals eat the carcass. This isn't discussed either, but it sounds like a) local wildlife aren't big on scavenging dead animals, and b) the wildlife losses from this would be significantly smaller than from the rat infestation.