He is not saying soundgarden is better then led zeppelin. That would be ridiculous. But I agree with him Cornell > plant.
Robert Plants singing was the worst part of Zeppelin (a little too much high litch screeching) but their music itself was unbelievable outside of the riffs they ripped off of black sabbath. I give Jimmy page the credit for writing most of the music though whereas cornell sang and wrote most of the music. Just my 2 cents.