There are several other buildings in the area that have also been supported this method(friction pilings). Yes, it is on landfill. So, is the entire BACK BAY section of Boston. Where the 55 story Prudential Center and the 60 story John Hancock building have sat since the 1970s. The difference being in Boston that the Charles River Back Bay had SOLID ROCK underneath it.
The original ground underneath the tower in San Fran is on the land fill from the earthquake that destroyed the city in 1905.
The original ground/bedrock was 200 feet below the current street level. So, they used friction pilings. A method that had been employed by many other structures in the area.
FYI, many parts of Manhattan are also landfill. That is why the really tall buildings are at the southern tip where the Twin towers once stood and from 23rd street(Empire State Building) and up through mid town. Greenwich Village buildings are only 4 stories high because the ground underneath there was wetland back in the 1600s. It is all landfill.
Eastern cities have much more room for error with their minimal earthquake risk.
This building is going down—a major earthquake will hit long before they are done tinkering with it.