I imagine the difficulty rating on the songs are the instructor's or group of instructors opinion of the complexity of the song. But one factor that the ratings can't address is your and my capability when we choose to sit down and learn a song. What is easy for you may be hard for me. I might be able to handle the strumming and rhythm of all the chords but be crap on the riffs. You might handle riffs but can't do a barre chord. Lots of variables...and a single icon of some number of guitars is not gonna address them all for all players.
Moreover, I think there's some weakness in the concept of searching for songs on the basis of someone's/anyone's opinion of the difficulty of each of the songs listed. If I looked for songs that match what I think my level of chops is there's a whole bunch of songs I'd miss and wouldn't be playing now. Instead, I always start a new song because I like the song. But all that is an opinion too. :D
"It takes a lot of devotion and work, or maybe I should say play, because if you love it, that's what it amounts to. I haven't found any shortcuts, and I've been looking for a long time."
-- Chet Atkins
-- Chet Atkins