I think the idea is that even in a vacuum there are inherent fluctuations in electromagnetic field. They're adding fluctuations that are comparable to the backg

I think the idea is that even in a vacuum there are inherent fluctuations in electromagnetic field. They're adding fluctuations that are comparable to the background but slightly different; to maintain conservation of energy the vacuum fluctuations have to shift in response (I think? I'm not an expert in this area). I guess one way of thinking about it is that at the quantum level there's no true vacuum in a classical sense.