From what I know, research does not support the idea that dragonbox is a particularly effective way to teach algebra. Which is too bad, because it seems like it would work really well:
I've seen first hand how well it works in a study with 40k kids in the US, Norway, and France. The study was led by the Center for Game Science, Washington University.
Paper: Zoran Popović. Achieving 96% Mastery at National Scale through Inspired Learning and Generative Adaptivity. Proceedings of the Second (2015) ACM Conference on Learning@ Scale, 2015. http://dl.acm.org/ft_gateway.cfm?id=2724684&ftid=1550376&dwn...
Post-test success as used in the paper you mentioned can be a misleading indicator if transfer to pen and paper was done improperly. As kids solving equations in a game are moving terms around by touch on a screen, they need to transfer that learning to pen and paper before being able to write a line by line solution. Failing to do this transfer would predictably lead to a poor post-test performance, without actually measuring properly how much algebra had been learned and understood.
See this 2012 thread on a truly awesome algebra learning game, and this stays relevant to this day. To the best of my knowledge, it is still the best and fastest way to learn how to solve an equation for x. https://news.ycombinator.com/item?id=4188579