The quote doesn't say one has peace in the law, it says that people have great peace when they love the law.
People can know the truth, but still be deceived. One has to love the truth, have a passion for truth, seek it like a treasure, and once found, never let it go. Then God won't send them a strong delusion, because they love the truth as a principle in the life, it isn't about just knowing a string of facts.
So when someone loves the law, they are loving more than just a list of commandments, they are loving the Law-maker, they are loving the principles embodied in the law and which were personified in Christ. They are loving the government which is founded on that law. They admire and appreciate the law as being eternal, a transcript of righteousness... The righteousness of God Himself. It goes far deeper and further than simple obedience. It recognises that the principles of them law are as eternal as is the God who spoke them. A God of love who is not arbitrarily being a demanding tyrant, but deeply cares how His people live... That they be happy, content, and yes, at peace with God and one another.
It's a deeply blasphemous insult to claim that God has done away with and discarded the very law that underpins His government and kingdom. It is no different than claiming God has cast aside His own character and become an anarchist.