He claims that the Covenant of Works has been laid aside. In other words, the Law still very much exists. But it is fulfilled for those who have fled to Christ for a refuge. To say that the Covenant of Works has ceased to exist is tantamount to saying that the Law is now void. If they are under no covenant at all, then they are guilty of no covenant violations.
Unbelievers are still under the Covenant of Works.
Bunyan's whole point in his book is that sinners must be apprised of what covenant they are under by nature, so that they would look away to God's mercy for another covenant. But, the fact is, the Covenant of Works is very much in force. In this way, Christ fulfills the eternal Covenant of Works for them. Especially a covenant that expresses His moral righteousness. The covenant of Works is an eternal covenant.