In the article, they were able to make more power with LPG, and also overcome the increase of fuel consumption by running the LPG in a similar fashion to that of a normal fuel injection system. Apparently the current common design for LPG systems is based on a design founded in the 1950's and is similar by comparison to a carburettor type set up, so as you can gather - is quite inneficient by todays standards.
Having said that, im not an expert on the topic, and am only re-iterating what I can remember from the article