I explained that they have to define a noun by its cognate verb meaning. In lexical semantics this is called the cognate fallacy. A word-study error to avoid! It is a form of the common root fallacy.
I also demonstrated an inconsistency in their reasoning in which they do not do the same for other noun forms of apostasia; but for this noun they have read their pretribulational preconceived views into this word in this context.
“Let no one deceive you in any way. For that day will not arrive until the rebellion comes and the man of lawlessness is revealed, the son of destruction.” (2 Thess 2:3 NET)