No, that's not what it means. What it means is, IF someone is kidnapped/forcibly confined, and they die as a result (hit head on dashboard or ANY kind of accident), the perpetrators are guilty of first-degree murder even without any intention to kill. In such a case, nothing about intention, premeditation, etc. needs to be proven. Thus, IF the Crown can make a forcible confinement scenario stick, BOTH the accused are guilty of first degree murder no matter what their intentions were, who fired the gun, or anything else. Guilty, period.
But if the forcible confinement issue is not proven beyond a reasonable doubt, the Crown needs to demonstrate that the murder was "planned and deliberate" (the Criminal Code terms for first-degree murder). Bringing a gun along on a supposedly innocuous venture such as a test drive is indicative of planning to do harm, if not necessarily murder, but being in possession of the gun, DM (or MS) could have then made the decision to kill and acted upon it if, for example, TB threatened the success of the "mission" in some way. There is no doubt the gun was used, and likely from the driver's side, and since DB was left handed it couldn't have been an accident -- drawing and firing would take deliberate and careful maneuvering.
Proving forcible confinement makes things simpler, but isn't necessary for a conviction, certainly not for DM. MS may be a different matter. I don't foresee an acquittal, but possibly second-degree. However, the evidence is not all in. We shall see.