Debugging is not always harder than writing the code in the first place (let alone "twice as hard" - what does that even mean)?
Debugging doesn't mean proving there are no bugs. If it did, the quote would be even more wrong, since our inability to do that has nothing to do with how cleverly code is written. But that's beside the point, since nobody uses "debugging" to mean this; it means tracking down and fixing bugs you know about. And it's trivial to see that that's not always harder than writing the code in the first place: some bug fixes (e.g. typos) are easy.
I guess I'm taking it for granted that any piece of code is going to have multiple bugs in it, and you're going to spend more time debugging than you did writing. I still don't think you've said anything that shows the maxim to be false in the general sense, only in the sense that it could be worded a little more clearly.
Debugging doesn't mean proving there are no bugs. If it did, the quote would be even more wrong, since our inability to do that has nothing to do with how cleverly code is written. But that's beside the point, since nobody uses "debugging" to mean this; it means tracking down and fixing bugs you know about. And it's trivial to see that that's not always harder than writing the code in the first place: some bug fixes (e.g. typos) are easy.