TJ originally hired Callender to print lies about Adams but since TJ was a chronic debtor he was unable to pay Callender for his "services". TJ also lied to his old friend Adams about his sponsorship of the slanders Callender wrote. When Jefferson refused to pay Callender, he retaliated by printing the TJ/slave story. DNA testing indicates a Jefferson male did in fact boink his slaves but it does not narrow it down to TJ himself. Given TJ's duplicitious deceptive nature, I wouldn't put it past him.
Jefferson brought this on himself.