Lehrer did have a tendency to insert "spoken interjections" in some of his songs during performances, so what's in the songbook might not match what's on one of the recordings.
The line in question looks like it might be one of those, but it certainly fits - no problem.
It does suggest that he's one of those for whom different sources might give different results, so source citations would be helpful(?).
I gave up long ago trying to track variations in recordings by one of the favorites at our big annual festival (Mike Cross) because he got tired of my favorite versions really quickly and kept changing them.