Creating a formal system, an artificial language, is not a mathematical discovery and it has nothing to do with "math things." There are interesting "math things" one can discover about Python, but no one would take you seriously if you say Python was discovered rather than invented, and no one who knows the history of logic would take seriously any claim that the lambda calculus was discovered. The person who invented the lambda calculus said it was invented in the very paper he presented it, so no, the people who say it was discovered do not know what they're talking about, at least not on that matter.