Jacob makes a very important distinction here. He declares that through God he has comprehended their thoughts. While he does not specifically note that this is in contrast to their actions, it is absolutely implicit in this statement. What Jacob is going to begin talking about is a sin of the thought and intent, not in the action. Jacob is not going to condemn the outward action but the inward thought of the heart that is manifest in those actions. In this he predates Christ's Sermon on the Mount, where this clear distinction between intent and action is declared. Jacob uses that distinction here.
These are people who are beginning to labor in sin - not because of what they are doing, but because of the way that their actions are altering their hearts.