I'll say it's most likely Bob on his guitar tuned, plectrum banjo. It does have that plectrum (flat pick) sound. As well as some of the guitar work on that wonderful song, is a bit more intricate than one would expect from Bob.

However it's not impossible for the 5 string to mimic the plectrum sound. Basically the plectrum started life as a 5 string with the drone string removed. Of course "Plectrum" is the hi falutton* word for ....pick. As in strum style.

* Spelling doesn't count :>)

If you'll recall the late Jimmy Henley, in Hee Haw appearances, he would often play both styles in the same song.