Thus being a fundamental question of constitutional law, this translates into asking how SCOTUS would likely rule given a certain situation where e.g. there was no confession and the two-witness requirement is not satisfied. In the case of Cramer v. US, 325 U.S. 1, the direct testimony of two or more witnesses established that "Cramer met Thiel and Kerling on the occasions and at the places charged; that they drank together, and that they engaged long and earnestly in conversation", but "There was no proof by two witnesses of what they said, or in what language they conversed; no showing that Cramer gave them any information whatever of value to their mission, or that he had any to give; no showing of any effort at secrecy, they having met in public places, and no evidence that Cramer furnished them shelter, sustenance, or supplies, or that he gave them encouragement or counsel, or even paid for their drinks". The ruling (in favor of the defendant) focused on the fact that what was suffiently witnessed was not treasonous (drinks and conversation are not overt acts of treason). The court assigns some significance to the testimony of a single witness Kopp, stating that
To the extent that his conviction rests upon such evidence, and it
does to an unknown but considerable extent, it rests upon the
uncorroborated testimony of one witness not without strong emotional
interest in the drama of which Cramer's trial was a part.
The fact that the evidence was uncorroborated is dispositive in this case.
There has not been a case where a conviction was supported only by circumstantial evidence, and the language of the Constitution plus the meaning of "testimony of a witness" is clear enough that it would be a major break with legal tradition to say that circumstantial evidence can substitute for direct evidence (testimony).
A video recording cannot testify, only a person can testify. A person can testify that they watched a video, but they cannot testify that they directly witnessed defendant making a certain statement. Rather they can testify that they inferred from watching the video that defendant made a statement. This is not to say that some SCOTUS could not find a path for conviction based on circumstantial evidence, but that would be a significant break from existing tradition.