Not necessarily true, more rehearsal isn't the key to fluent oratory.

Many people can speak off the cuff fluently and confidently, avoiding "like", "um", and other filler words. And even if you're not speaking fluently, leaving silences as punctuation is more effective, IMO.

Many impressive speakers I've met actually cite Toastmasters! So their obsessive zeal actually does work.

More rehearsal does work too sometimes, but it does sometimes lead to speeches "sounding too rehearsed".