You are correct. It was just a stupid chant used to incite anger and revenge feelings within a mob of thugs. That’s what got him off - everyone cheering and chanting.
Trump would often admit in interviews that he used the “Build the wall” mantra because it got people so hyped up. Not because he ever intended to build a wall.
Chanting “Lock her up” and its more grisly variations were a way of his thugs letting off steam.