[lordnacho]

Yeah isn't this one of those logic things?

Perhaps what they mean is NotB -> NotA, which often uses a symbol that maybe is being erased?

In any case the abstract seems wrong.

[DebtDeflation]

Yes. Modus Ponens vs Affirming The Consequent.

If A then B. A. Therefore, B. -> Valid.

If A then B. B. Therefore, A. -> Not valid.