I think you may have missed what I think is the most important point made in the article:
"It is also interesting to consider that students in the treatment group spent approximately half of the summer working on pre-algebra skills. Because the Khan software is individualized, it identified that most of our students had significant pre-algebra skill gaps and delivered instruction and practice problems to address these deficits. Students in the Khan/treatment group therefore spent up to 50% less time than the control group on the algebra content that the MDTP exam measured. The treatment group, however, still performed at a similar level the control group on the algebra measures."
That says to me that these students spent most of their time filling in gaps in their knowledge and still managed to keep up and even get ahead of the other class in a very short amount of time. They're basically saying flat out that these students learned as much of the advanced material in half the time as the other group and probably have better foundational knowledge.
"It is also interesting to consider that students in the treatment group spent approximately half of the summer working on pre-algebra skills. Because the Khan software is individualized, it identified that most of our students had significant pre-algebra skill gaps and delivered instruction and practice problems to address these deficits. Students in the Khan/treatment group therefore spent up to 50% less time than the control group on the algebra content that the MDTP exam measured. The treatment group, however, still performed at a similar level the control group on the algebra measures."
That says to me that these students spent most of their time filling in gaps in their knowledge and still managed to keep up and even get ahead of the other class in a very short amount of time. They're basically saying flat out that these students learned as much of the advanced material in half the time as the other group and probably have better foundational knowledge.
No significant difference my ass.