Oh, I picked it up from Church's original 1936 paper. Those were direct quotes. Did you actually read Church's An Unsolvable Problem of Elementary Number Theory and Turing's On Computable Numbers? If so, could you point out Church's rigorous definition of computation?

BTW, while the lambda calculus does indeed precede Turing's work, its recognition as a universal expression of what is calculable happened at exactly the same time. But regardless, Church does not define the notion of computation, while Turing does.

But you don't have to take my word for it. You can find such rubbish by Gandi (who called Turing's work a "paradigm of philosophical analysis"), Davis, Gödel and many others: Gödel offered enthusiastic praise when he wrote that Turing offered "the precise and unquestionably adequate definition of the general concept of formal system" (Floyd, https://mdetlefsen.nd.edu/assets/201037/jf.turing.pdf)

The belief that Church defined computation rigorously when he solved the Entschidungsproblem using the lambda calculus rather than just made an imprecise conjecture is a common mistake and historical revisionism. If anyone came close to defining computation before Turing, that would have been Brouwer.

Also, I don't know if Turing worked on lambda calculus before he wrote On Computable Numbers. After completing the manuscript, he saw Church's new paper and incorporated it in an appendix. Do you have any reference for that?