The prophet did not have to be told that he was dirty, he knew by looking into glory, seeing the Lord on His throne, His long train filling the temple, angels and seraphim singing all around. The guilt/shame was sensed in His lips. The words going out, the appetite going in, the gestures of affection are all in the lips. It would be wise for us to consider his physical awareness of sin as we consider ours in light of the Lords full glory.
It is amazing how differently Jerusalem will be viewed and treated in the age to come. How many centuries has it been hated and trampled. Yet it is the sign of His covenant. When He finally glorifies it it will be truly glorified. Other peoples will bring from their treasure to make it the delight of nations.
It may seem odd that there is such a focus on singular objects, Jerusalem and Zion in the age to come. Surely the focus will be on the Lord, but, the tangible proof of that focus will be these symbolic things. That the Lord has done this, that it is now viewed by others as He had said it would is the proof of His commitment. That we would come to do this in this manner is then proof of ours.
I cannot explain how a Hebrew today would be able to explain away the fact that their Lord was found by another people and not them because of their rebellion. It is said so and has been said all along. How does one explain this passage if it doesn't say what it says. It is not as if we went out seeking to replace them as the apple of His eye, it is that sought us out and found us because they wouldn't listen.