Thomas did not claim that Covid Vaccines were made using fetal tissue.. He was just summarizing the chain of events. And part of the chain of events was that the people suing New York for the mandate were doing so on religious grounds claiming that the vaccines were made with fetal tissue which is against their religious beliefs.
>Petitioners are 16 healthcare workers who served New York communities throughout the COVID–19 pandemic. They object on religious grounds to all available COVID–19 vaccines because they were developed using cell lines derived from aborted children.
https://supremecourt.gov/opinions/21pdf/21-1143_3f14.pdf