And how does that upend anything? That it fell out already in the car does not seem dramatically different from it falling out after shaking Kennedy a bit.
If the story in the article is true, the bullet Landis found - i.e. the famous "magic bullet" - could not have wounded both Kennedy and Governor Connally and have been shot from the Book Depository (to Kennedy's rear) as the so-called single-bullet theory[0] requires, so it upends the Warren Commission's version of the assassination completely.
* https://spacecoastdaily.com/wp-content/uploads/2017/11/JFK-A...
look at the arms. Kennedy is resting his arm on the car frame and Connally cannot. The seat he is in is not lined up with where Kennedy is. The bullet needs no magic to go in a straight line.
So if true (not that we can really know at this point) if the pristine bullet was retrieved from Kennedy's car then yes, that would upend everything.