In this context, the meaning is close to unadulterated. A good wine should not be mixed with inferior wine. An old masterpiece should not be modernized during a restoration.

The word ’integrity’ has various meanings, all of which relate to ’wholeness’, authenticity and honesty. A person with integrity is an honest person who does not deceive others, and in a similar sense, an artwork has its integrity preserved if it is seen in its original form and context. I suspect that the reference you give concerns attempts to preserve the original appearance of old paintings, etc, and not to use techniques that might alter this.

Not only that. Integrity is often used as a synonym for being genuine. So it might mean the artistic intentions of the piece. If you take a piece of graffiti off a building by removing part of the wall and then display that in an art gallery there's a possibility that you lose some of the artistic quality because it's no longer in the setting the artist envisioned.

You got it correct.