Hmm ... given that 'can' for 'toilet' dates back to the 1950s (www.alphadictionary.com) and the song is found printed in a 1609 songbook, I don't think that theory holds water (pun intended).

I don't think song is simply about 'impossible things' per se, like, say, The Derby Ram. Rather, I see it as being about hallucinating while drunk.