I see by "first" they meant the first evidence of vacuum birefringence. Quantum vacuum fluctuation is by itself well established. E.g., Casimir effect.
Thanks for the reference. Just to be clear, his point is made with regard to the reality of vacuum zero point energy, esp. whether it should affect the cosmological constant. However quantum vacuum fluctuation (which I read as meaning the virtual particles) is essential to QED whether zero point energy is real or not. As far as I can tell, quantum birefringence also does not refer to the zero point energy.