That’s not an original source to prove the claim in the headline. It’s someone making a claim that the DoJ is making that claim. (A video of “B says that A said” is not the same as an original video or text from A, particularly when A and B are adversaries.)

He is an original source, as a person who saw the filings. Perhaps not unbiased though. And contra your statement, there is no reason to suspect that the DoJ's self-description of their filings would be in any way more accurate. (Do you trust the student's description of his essay, or the teacher's evaluation of it?)

The DoJ filing is an original source for the argument. The video above is much closer to hearsay than to an original source. I would no more (and no less) accept hearsay as original testimony from counsel on either side of the case.

There is some difference between "testimony" and "reporting". One is law, one journalism. There are, quite rightly, different standards.