It seems rather incredulous...
I think "incredible" is the word you want. People can be branded as incredulous. Ideas can be branded as incredible.
Heck here’s the actual testimony of one of the shooters who managed to pull off the shot. You might be interested to learn the WC listened to him...
How does he stack up against Hathcock as an expert?
"Retired Gunnery Sergeant Carlos Hathcock is likewise
skeptical of Oswald's alleged shooting feat. Hathcock is a former senior instructor at the U. S. Marine Corps Sniper Instruction School at Quantico, Virginia. He has been described as the
most famous American military sniper in history. ... Craig Roberts asked Hathcock about the marksmanship feat attributed to Oswald by the Warren Commission. Hathcock answered that
he did not believe Oswald could have done what the Commission said he did. Added Hathcock:
"... We reconstructed the whole thing: the angle, the range, the moving target, the time limit, the obstacles, everything. I don't know how many times we tried it, but
we couldn't duplicate what the Warren Commission said Oswald did". (KILL ZONE, pp. 89-90).
IF Hathcock really said that, and if Craig Roberts is really who
he claims to be, some people might trust what they say about Oswald's shooting abilities more than what some anonymous chair-dweller in an internet forum says that contradicts them.