Elon was serious about the debt. Thats why he and Trump don't get along any more. After the initial DOGE efforts, Trump raised the debt ceiling a few trillion dollars and got a new spending bill passed that increase spending like another trillion dollars - obviously not concerned about the debt.
"serious" at a superficial level. He never understood what he was doing, and went around smashing things indiscriminately, often in a way that was more expensive (e.g., laying off federal workers who were paid for doing nothing for months, then rehired when someone realized that they were in fact doing something valuable).
Why do you think otherwise? He was very clear about it long before DOGE. He viewed the debt as an existential threat to the country and believed Trump was the only candidate that might actually fix it. Looks like the joke was on Elon.
I just don’t believe Musk has any ability to reason about things from a rational perspective. Some of the best work done to address the US debt has occurred under democratic leadership, while time and again we’ve seen republicans balloon the debt.