I argue the real problem was in management - two guys working alone and the rest of the team not participating and not learning until late in the game. Adding people to the project at the end of its lifecycle rarely works well. No knowledge sharing and one guy leaves, what can go wrong?
Another problem is not having anything working for a very long time in the beginning. After a week of work the management should have asked for a demo and if nothing was working, they should start questioning the skills of those two „rockstars”.