Right, but look at cercopithecus, an example of which is the Vervet monkey.

They are modern monkeys and they also fall far away from the regression line.

There are several old world monkeys which are roughly as far below that line as the australopithecines, just as there are several above the line. The line just represents the mean in terms of brain volume by blood flow; australopithecines would fall below the mean of the modern great apes - that's what the study reports.