He is essentially saying what I have being saying...nothing, as per your definition is impossible and has not nor ever did exist....or as aptly described by the following...
Thorham...I'm no scientist, nor philosopher. In fact like Krauss, I have drawn the ire of philosophers at times, when quoting such lines as "Science is what we know: Philosophy is what we don't know" In actual fact I don't like being labeled anything, other then hopefully, a lover of reason and the scientific methodology, as corny as that may sound. In reality I'm an old retired maintenance Fitter/Machinist/Welder that loves to learn what I can about physics and cosmology, and sometimes regretting that I left it too late in life to do anything about, other then badgering those more knowledgable then myself on forums such as this.
I see it as awesome that cosmologists can take us back to t+10-43 seconds with some confidence, and relay with logic how things progressed from there. I'm equally in awe, how we can also speculate with some degree of knowledge and data, how scientists like Krauss for example, can even delve into the abyss of whatever nothing existed before space and time, as we know them. I find it far more logical and reasonable to accept such speculation as the quantum foam, then other unscientific solutions, that first arose with ancient man.
I admire the logic and science of scientists such as Krauss and Dawkins, but my most respected scientist, and probably the one that got me all fired up and into this stuff is the late Carl Sagan. I have his "Pale Blue Dot" narrative hanging up in my man cave.
Anyway nice debating with you and look forward to more when such time may arise.