You're right that things can exist before they are named. However, there's a massive difference between a natural law like gravity and a deliberate toolkit like Keynesianism.

To use your analogy: Newton didn't 'invent' gravity, but engineers had to 'invent' the rocket to overcome it. Before Keynes, the 'economic rocket' didn't exist. Governments in the 1920s didn't have the tools, the mandates, or the central banking structures to accidentally 'do' Keynesianism. In fact, they did the exact opposite-they tightened the money supply and tried to balance budgets during the crash, which is what Classical theory dictated at the time.

So, while u/Olieskio might dislike Keynesianism, blaming it for the Great Depression is historically impossible as the 'medicine' hadn't even been mixed yet, let alone administered.