The thread title begs the question (I believe that's the right term - JayUtah can correct me if it's wrong). By saying that Armstrong would "admit" the hoax, it implies that he would be telling the truth - one does not "admit" a lie. Therefore, disbelieving him would be refusing to admit the truth.
However, what would (hypothetically) happen would be that Armstrong would "say" there had been a hoax. This is neutral, and would still be up to the listener to determine if what he said were true or not. For example, many people in their old age might develop mental issues leading to delusions or hallucinations. If Armstrong, before he died, said "I never walked on the Moon," that could easily be untrue, even if at that point he believed it was true.
So, the question is, what support could he give? If it were an old man mumbling to himself, no, I wouldn't believe it. He could be delusional, he could have been pressured, he might be telling a tall story just for the heck of it.
If it were Armstrong still mentally strong and able to describe exactly how the hoax was perpetrated, then I would have to take that seriously.
Many people over the years have "admitted" to being involved in the Kennedy assassination, but their stories are contradictory. I believe none of them without additional proof, because talk is cheap. Evidence is valuable.