I disagree that we shouldn't
question such big claims.
It's safe to assume the reason his sources are unnamed is to protect their safety. Don't know how plausible this is, but I it's possible that the lack of presentable evidence is for the same reason. Maybe the relevant documents could've been somehow fingerprinted, which would identify the leaker/source; the film/tv industry has done this when distributing pilots for private viewings. Heck, even printers did it lol.
However, it's not secret American politicians vehemently disliked the existence of Nordstream, and this outcome is undeniably convenient for them. Maybe too convenient, so they wouldn't dare attempt it? Or maybe they just assumed they'd have a great scapegoat. Maybe it wasn't even them, and it's Russian government playing 5D chess by blowing up their own investment to frame Americans.
Who knows? Maybe time will tell; it usually does.