And yet numbers like that have been talked about for thirty years, so why is a public admission of what has been generally known hard to swallow?
Because these types of incidents tend to get overinflated. Were you in Viet Nam? I can assure you with 100% certainty that the reported body counts were inflated - dramatically.
I also have an issue with the fact that an operation like this involved thousands of soldiers. Many of whom are old men. Old soldiers often get regrets and I have yet to see one story that confirms the OP from a soldier on his death bed.