The bullet point you object to doesn't claim to "refute" the WSI, it just notes what Lewy had to say about the report. That Lewy had access to classified materials is mentioned, among other places, in "Stolen Valor" on page 133:
"The results of this investigation, carried out by the Naval Investigative Service are interesting and revealing," said historian Guenter Lewy in his book America in Vietnam. His history of the war was one of the first to rely on previously classified documents in the National Archives. "Many of the veterans, although assured that they would not be questioned about atrocities they might have committed personally, refused to be interviewed. One of the active members of the VVAW told investigators that the leadership had directed the entire membership not to cooperate with military authorities."
One black Marine who testified at Winter Soldier did agree to talk with the investigators. Although he had claimed during the hearings that Vietnam was "one huge atrocity" and a "racist plot," he could provide no details of any actual crimes. Lewy said the question of atrocities had not occurred to the Marine until he left Vietnam. His testimony had been substantially "assisted" by a member of the Nation of Islam.
"But the most damaging finding consisted of the sworn statements of several veterans, corroborated by witnesses, that they had in fact not attended the hearing in Detroit," Lewy wrote. "One of them had never been to Detroit in his life." Fake "witnesses" had appropriated the names of real Vietnam veterans.
Lewy pointed out that incidents similar to those described at the Winter Soldier hearings did occur. "Yet these incidents either (as in the destruction of hamlets) did not violate the law of war or took place in breach of existing regulations," Lewy wrote. Those responsible were tried and punished.
But the most damaging finding consisted of the sworn statements of several veterans, corroborated by witnesses, that they had in fact not attended the hearing in Detroit," Lewy wrote. "One of them had never been to Detroit in his life." Fake "witnesses" had appropriated the names of real Vietnam veterans.
Now this would really do some damage to the WSI, if confirmed. Unfortunately, we only have Lewy's word for it.
I can't see how it would violate security to release the names of the imposters, or at least the names of those who had their identities stolen.
Imagine the testimony of a guy saying someone had stolen his identity for the WSI, and a call comes for tracking down the imposter. Broadcast on "America's Most Wanted".