While it is correct that one can make an untrue statement for a number of reasons, without necessarily being a "liar", it is also the case that once a person is informed of an untruth, continuing to make the untrue statement is generally considered "lying".
In short, I agree with you that TRiG was premature in calling disciple a liar, but he did later qualify that by saying that if disciple did not know it was an untruth, s/he was simply ig-norant.
That's actually not true. I can't speak for Scotland or England, but here in the US, ignorance of the law is not considered a valid excuse in a court of law.A cousin of mine from England visited me here in Glasgow, And we had introduced the smoking ban a few months earlier - My cousin lit his cigarette, A criminal offence. He committed a criminal offence - without knowing that is what he is doing. Every lawyer in Scotland, England and the United States would tell you
"This man is not a criminal"