From the first line of the opinion, "We granted certiorari to determine whether a process for curing synthetic rubber which includes in several of its steps the use of a mathematical formula and a programmed digital computer is patentable subject matter under 35 USC 101." Diamond v. Diehr, 450 U.S. 175 (1981).
Rehnquist, who wrote the majority opinion, said it right there we are talking about "formula + computer."
Claim 1 of the patent itself reads, "1. A method of operating a rubber-molding press for precision molded compounds with the aid of a digital computer, comprising..." Diehr at Footnote 5. The formula is the Arrenius equation (everyone already did this), and then Diehr added a computer for continuous monitoring, calculating, and output (the novelty).
Diehr themselves claimed in their arguments that their novelty to is "the continuous measuring of the temperature inside the mold cavity, the feeding of this information to a digital computer which constantly recalculates the cure time, and the signaling by the computer to open the press..." Diehr at 178-179.
EDIT: You edited your previous comment to include the same line as me. It seems as though we actually agree on what was novel here.