Factual error: The Boccherini string quintet is actually scored for 2 violins, 1 viola and 2 cellos, not the crooks' line-up of 2 violins, 2 violas and 1 cello.
Suggested correction: I don't really know anything about how string quintets (or quartets) perform, or how any group of musicians might prepare to perform a well-known piece composed many years ago. So maybe my answer is not valid. But would it be acceptable within the music community for a musical group to re-score a piece, so long as they remain faithful to the original? If two members of a string quintet said they were more confident and capable on a viola than on a cello, would the quintet re-score the piece accordingly? Is it acceptable for musicians to adjust a long-established piece of music? Another possibility. The crooks are not musicians at all. They pretend to be musicians, but this is only a "front" for a bank robbery they are planning. They maintain the pretence by miming to records. So is this a subtle joke?"Look, the crooks are pretending to be musicians, but they are getting it all wrong! They are playing the wrong combination of instruments. Ha ha!"
I asked a friend who belongs to a string quartet if you could re-score a quintet piece. She said this would be complicated, difficult, and more trouble than it was worth. So I must retract my suggestion that the crooks re-scored the Boccherini String Quintet. But maybe I am right with my other suggestion. The crooks are not musicians, but only mime to records. Thus, they use the wrong combination of instruments. This is therefore intended as a subtle joke.