Wrong. Remember that Tom and Diana received a check from the insurance company in exchange for that original bill of sale, and no doubt also signed a release against further claims in exchange for that check. Should they now sign a second bill of sale transferring ownership of that plane to another party, there could be both civil and criminal consequences -- you can't legally sell the same item to two different people.
They have not been the legal owners since they signed the original bill of sale and gave it to the insurance company in exchange for a check settling their insurance claim. Whether the insurance company registered that Bill of Sale with the FAA or not, that Bill of Sale is the effective title to the aircraft any time the insurance company chooses to file it with the FAA. If Tom and Diana sign over a second bill of sale to a third party, they will almost certainly be breaking the law.
For those legal eagles out there, I am well aware of Dowell v. Beech Acceptance (11 Avi 17,491 Calif. Dist. Ct. Appeals; 11 Avi 17,831, Supreme Court of California). It is probable that if the insurance company never filed their Bill of Sale with the FAA, and Tom and Diana give the new party another bill of sale, and the new party files that new bill of sale with the FAA before the insurance company files theirs, and there is a contest between the insurer and the new party, the new party may well prevail as the owner of the plane. However, if the new party has actual notice that title was previously conveyed to another party, the first party may win (Shacket v. Philko, 21 Avi 17,273). In this case, even though it appears the insurer never filed their bill of sale with the FAA, it's clear that the new owner knows that Tom and Diana already conveyed title to the insurance company, and so the insurer may still prevail; the courts would have to sort this out.
However, if that contest does develop, Tom and Diana having accepted a payment from the insurer in exchange for that first bill of sale, they would likely face fraud charges for selling the same plane to two different parties, as well as a lawsuit for damages by the insurer (wanting back their payout, plus interest, plus expenses). The bottom line is that you must actually hold title to a plane in order to convey perfect title to another party, and once you deliver a valid bill of sale to someone, even if that someone fails to record that bill of sale, you no longer have title to that plane, and you can no longer convey title to third party.