Incorrect. If an audio track is present, the mp4 does not qualify for autoplay unless other conditions are met.

EDIT: Context is unclear about whether the parent's guidance is about <video> or <img>. No idea what <img> does. I sure hope it doesn't play due to the wasteful audio track, so that people are forced to do it right instead of lazy, though!

in the context of <img> the audio track doesn't play anyway. dropping it ensures you save the bytes.