I think what you mean is "any efficient engineering process will make good use of testing". But if you already have some tests in place, the most efficient way

I think what you mean is "any efficient engineering process will make good use of testing". But if you already have some tests in place, the most efficient way to reduce bugs is often something other than more testing, e.g. code review, design review/improvements, or static analysis (often provided by the language/compiler). All of those other methods can and will reduce bugs. So your statement that testing is the only way is obviously false in both a practical and theoretical sense.