I don’t know the truth of the matter and Seymour could be right. We just can’t tell from the evidence provided.
This pipeline had been completed but activation was stopped due to the start of the war. If Germany had capitulated and activated it with Russia that would be a major political win for Russia and blow for the suggested goals of the US and other allies. To me this seems to be the biggest hole in the theory that the US was responsible.
Two possibilities:
(1) explosive charge malfunctioned
(2) make plausible arguments that it was Russia who did it