As mentioned in the above article, the video was in fact taken at the first rehearsal. The lady could have stopped the rehearsal and asked for the sheet music - expensive minutes would have been lost, that's all the risk. She decided that she remembered the concert well enough, having played it countless times.

Most solo concertos are such that learning them by heart is the least of the difficulties, and found advantageous.

The PR aspect comes in when the agent decided to publish the material (or to allow its publication), possibly filmed by a viola player as a private souvenir - they have nothing else to do, have they?