有这样一个句子（在百度上可查到）：

He sang much more sweetly than he has before. 他的演唱比以前要动听许多。

不明白句子中than he has before的时态。按理说，这里的 than he has before 应该是than he has sung before的省略，但无论从哪方面来看，这都有问题：从主句时态sang来看，than后面的时态应该是过去完成时，所以应该是than he had before；从句末的before来看，也应该是过去完成时，即用than he had before。请问这是不是有than he had before的笔误？