They very well could be. I can't find any clear information on exactly what strings he was playing at the time, but here's a snippet from
an interview he did for MusicRadar:
Your bass sound was different after Sabbath’s debut album in 1970. What was going on?
“I probably changed the strings on my bass! On the debut album I was too poor to buy new strings, so they were always, like, 10 months old. I’d only replace them if they broke, so that’s what gave the record that dull, flat thud sound.”
This was just a couple of years after Rotosound rounds became popular in the British rock scene, so it's hard to say for sure what he would have been using at the time. But he does attribute his dull thuddy sound to the age of the strings rather than the construction, which suggests roundwounds. I really am just guessing at this point though.
Now though (since 2000 at least) he uses DR Black Beauties because they don't give him finger blisters.