I wonder if they included sexy images in their negative training sets -- many videos accrue millions of views (and ad dollars) by having a few frames of cleavage interspersed with other (often derivative) footage. It would be great if their algorithm picked a thumbnail that reflected the entire video, not just a few frames specifically chosen to game people's compulsive clicking.
They stated that the negative training set was constructed by randomly sampling frames from the video. If someone wants to game the thumbnails, then they will just manually select the thumbnail to use; and there are to many legitimate use cases for this ability for Youtube to remove it.
Many channels I watch carefully select an iconic frame from the video to serve as the thumbnail, or construct an artificial thumbnail that provides useful information about the type and subject of the video. Manual will frequently produce better results than automatic for a good-quality channel.