They've got to be training on more inputs than mentioned. For example, is one or a close set of times in the video linked externally and generating traffic? Grab the entire set of frames from that time period and run it through the quality classifier, there might be iconic frames from that section that people are looking for.
Are people re-watching a small segment of the video? Try classifying individual frames from that segment or just before. Of course, those are often action moments that result in smeared motion and artifacts and may not result in a quality thumbnail.
These ideas also only come into play when a video has been live for a while, after the uploader has initially picked a thumbnail. Maybe a "We have some new thumbnail suggestions for you, take a look" alert or message?
Are people re-watching a small segment of the video? Try classifying individual frames from that segment or just before. Of course, those are often action moments that result in smeared motion and artifacts and may not result in a quality thumbnail.
These ideas also only come into play when a video has been live for a while, after the uploader has initially picked a thumbnail. Maybe a "We have some new thumbnail suggestions for you, take a look" alert or message?