They are not for sure, but they have neither invented the plot. We do not declare the laws of physics fraudulent because Hitler's V2s used it.
You may dismiss Tgambill's sources but you can not dismiss the facts recorded in the official US Congressional records:
MacCormack-Dickstein Committee documents published by the House UnAmerican Activities Committee (HUAC):
Investigation of Nazi Propaganda Activities and Investigation of Certain other Propaganda Activities Public Statement of the Special Commmittee on Un-American Activities - House of Representatives, 73 Congress, Second Session at Washington DC, Nov. 24, 1934 (pages 1-12)
Investigation of Un-American Activites: (part 1 of) Testimony by Gerald C. MacGuire (pages 39-47)
Investigation of Un-American Activites: (part 2 of)
Testimony by Gerald C. MacGuire (pages 47-87), Testimony by Claude M. Adamson (of Central Hanover Bank) (pages 89-99) and (part 1 of) Resumed Testimony by Gerald C. MacGuire (pages 99-105)
Investigation of Un-American Activites: (part 2 of) Resumed Testimony by Gerald C. MacGuire (pages 106-125), Testimony by James Haggett (of Lawyers County Trust) (pages 127-128), Testimony by public accountant James Rempe (pages 129-136) and Testimony by Albert Christmas (Attorney for Robert Clark) (pages 136-162).
Any good librarian worth his or her salt can pull it out for you.
"You may dismiss Tgambill's sources but you can not dismiss the facts recorded in the official US Congressional records:
MacCormack-Dickstein Committee documents published by the House UnAmerican Activities Committee (HUAC):
Investigation of Nazi Propaganda Activities and Investigation of Certain other Propaganda Activities Public Statement of the Special Commmittee on Un-American Activities - House of Representatives, 73 Congress, Second Session at Washington DC, Nov. 24, 1934 (pages 1-12)"
...that's where I stopped. HUAC was formed in 1938. Period.
Or, End of discussion, as the guy you're shilling for might say. Watch the condescension, okay? And check things out with "any good librarian worth his or her salt" WITH ACTUAL CONGRESSIONAL RECORDS PRINTED DURING THAT ERA, not stuff ginned up on whackjob websites.