It’s “never drop the slide on an empty chamber”…and as far as dropping it on a live round, I believe that was in direct reference to dropping it on a round that had been manually chambered, not stripped out of the mag (dropping the slide on an already chambered round is a great way to bust an extractor on a 1911–or an XD with an internal extractor, for that matter).
The proper way to load 99%+ of autos is to lock the slide to the rear, insert a loaded mag, and drop the slide, either by the slide stop release or by retracting the slide and fully releasing it.
And, considering that the top 1911 gunsmiths are all in agreement in this…I’ll put money they know a little bit more on the subject than most folks—myself included. Which is why I take their advice.