Or something else?
It sounds as though these researchers have gotten a little carried away with their speculation based on their observation.
As far as I am aware cognitive behaviour require a fair bit of processing ability; this sounds more like evolved behaviour patterns.
When the trout has found prey and is unable to reach it, perhaps it has a particular behavioural trait which over time the eels have come to equate with the presence of prey that they can reach. Clearly the trout are not a threat to the eels so they are just using a form of opportunism as a hunting/food sourcing ploy. Obviously if the eel chases the prey out of the crevice before it can eat it, the waiting trout will grab it.
Where was it they had a jaunt to, to indulge in this shaky science?