IMO that's an odd distinction between heist and robbery. They both usually devote the first half of the movie planning the big heist, and that builds the suspense when things don't go to plan. I'm interested in seeing how the characters deal with and overcome adversary. I guess some of the suspense is if the characters can go under the radar and go undetected, but that doesn't define the genre to me.
I guess I'm curious of what are some heist movies by this definition? Like is Oceans 11 not a heist movie, because the casino owner knows he's being robbed at the time? Or Inside Man? These are quintessential heist movies.