I've heard (maybe read) Mark Hamill tell the same story, but it was Mark Hamill himself who filled the extras in on what was happening, which would make more sense than Guinness's being around to do so since (as you point out) he's nowhere involved in the scene except for the Force voice saying "Remember… the Force…

Also, I… need to think of a funnier AV Club commenter name than my actual name.

I feel horrible writing this but I always heard it as "farts," which I knew made no sense, either grammatically or as something someone would put into a song like this. Thank god for the internet for finally giving me the lyrics some ten years ago. I love you, internet!