Which is answered in the video - which is we don't know.

What we know is that there is no "nothing" as we understand or define it.

At the end of the video he says "We can't prove that we came from nothing, But it's plausible." from our limited understanding of whats going on in empty space. I don't know why you think "Krauss is often mocked for" this - because this is not unique to Krauss personal interpretation but there is scientific consensus within the community.

We can't tell where quantum fields came from the same way we can't tell for sure what happened before the big bang - other than the fact that we know it happened.