Well maybe their statement should be revised to say 'generally' they don't overbook flights! I believe there are very few situations in which they do, certainly an extreme minority in terms of total flights.
And of the few situations where they do do it, you would be very very unluckly to actually be denied boarding, even at that you'll be put on another flight on the exact same route on the same day. Hardly that unreasonable in the overall scheme of things, and certainly not something I'd be getting too worked up about!