Thanks for your replies.
However, despite the possible use of these other "signals", it appears to me that Google has made a serious error in giving this video a number 3 ranking, because it doesn't contain any information relevant to the search term.
I believe this example may reveal a major flaw in the way Google ranks videos. since it is apparently giving many of them high rankings without knowing what is actually on them.
To illustrate the risk in this approach, imagine a hypothetical case in which someone creates a #*$! video and gives it the title "Alice in Wonderland". Also imagine that this person embeds this video on a page called "Alice in Wonderland", creates external links with that anchor text, etc. Then unless Google has a way to directly check its content, a child searching for "Alice in Wonderland" might see this #*$! video at the top of the SERPs and click on it.
In other words, if Google is merely using titles and "external signals" as the basis for its evaluation, in some cases this may not be adequate to determine what is actually on a video. And if they don't know what's actually on it, they are taking a major risk in giving it a high ranking.
Google would never give an ordinary web page a high ranking without dirctly checking its content? So why is it giving videos high rankings without directly checking their content?