The only people talking SSTO for the upper stage (Starship) were speculating about stripping the landing gear and everything else related to coming back to Earth, to lighten ship. They were thinking of using these stripped ships as habitat modules or strictly for transfers between planets. It might be just possible with no payload and running the tanks dry.
The booster runs 31 engines to put the Starship up forty miles after a four minute burn. Booster and Starship are both reusable.
Elon very definitely stated that the Starship would SSTO, but had to concede that they’d go up and come back with almost nothing aboard.
Zubrin’s critique of the Mars colonization plan is valid — given the time frame of a round trip, making every craft just a transport vessel doesn’t make economic sense. Using AI to build the settlements on Mars, to include large indoor facilities to grow food, so that the colonists have somewhere to move to, makes sense, and would not require vessels making round trips, be a good way to start.