Ben is referring to the "mega tsunami" hypothesis, where a giant landslide could theoretically generate an immense wave. There are several reasons to doubt this idea.

In order to generate such a giant wave, the landslide would have to behave as a single coherent block. However, actual landslides don't tend to happen that way. While there is evidence that blocks have slumped off of the Canary Islands in the past, they seem to have broken as a series of subunits, which do not generate tsunamies on anywhere near the scale of a "mega tsunami".

Also note that a mega tsunami of that scale would leave a deposit as evidence. All that stuff scored off of Florida would get dumped elsewhere. Such deposits HAVE NOT been found from previous slumps of the Canary Islands.

See Killing off the Canary Islands mega tsunami scare for more info.