That's completely wrong. Reformat just deletes the index to the files, not the files themselves. However it is indeed easy enough to "wipe free space" with CCleaner or similar, which overwrites everything, making it unrecoverable.
I once bought an external drive from a jumble sale for £1. I undeleted it and found it was full of fascinating magic trick explanations and photos from some professional magician. Obviously computers were not in his field of expertise.