To set the record straight....From "Bass Heroes" a collection if interviews and articles from Guitar player (yes before there was a Bass Player mag). **
Q:What is your Bass Playing technique?
MM:When I began playing the electric bass my approach to the instrument was to play with my thumb because technically I was after the down stroke. I played the electric as if it was an upright, and it came out in the music.
Q:Why the Thumb?
MM: I have never played with the fingers. There is a different effect going down than coming up. The feel is different, attack is better. If you play with your thumb, you can't come up like you can with a pick. The downstroke came to me naturally, and I just would strike the string that way. I had no examples or influences here. I was an innovator on the electric. The stroke is used in guitar but that is a different instrument.
He later used a felt pick because his thumb stopped working, dubbing it the 'Bionic Thumb'.
**
Amazon.com: Bass Heroes (9780879302740): Tom Mulhern: Books
A very interesting book for those seeking the truth about the early days of bassdomness.