I actually agree with you about the complimentary relationship between imagination and knowledge, their "opposition" is more like the Yin and Yang if anything.
To defend Einstein's point of view a little, however, we have to keep in mind the field he was indulged in, and the very knowledgeable people he was running against. I believe that's what he said after he proved his -preposterous at the time- ideas, and very imaginative view to be correct.
It was his way to express his triumph over his peers who were considered more knowledgeable than him.