He didn't explain HOW "HTML5 videos are more searchable and indexable."

Is there an attribute or property that search engines pick up that isn't available when you embed flash?

A lot of the best additions in HTML5 are in the DOM. Search engines don't run JavaScripts so it doesn't seem like HTML5 adds much in the way of on-page optimization. Maybe the new section tags will have a benefit but this article wasn't about those tags.