How does pulsing at a frequency that's unrelated to the AC line frequency but still low enough to be visible improve efficiency? SMPS efficiency usually peaks in the hundreds of kHz, which is invisible even to the most sensitive observers.
You said "LED flicker usually has nothing to do with AC power." But most LED lighting flicker is the result of using a capacitive dropper power supply. These are cheap and efficient, but produce a lot of ripple at double the line frequency (with full-wave rectification). Filtering the output enough to make flicker imperceptible would cost more, and not everybody notices the flicker so it's easy to get away with cutting corners. See:
