I got a bug similar to this one but after investigation boost module turned out to be the culprit.
Whenever your data exceeds boostThreshold, csv and excel exports return empty data sets. Internally getDataRows seems to return only two rows then.
It happens both in my project and your jsfiddle: http://jsfiddle.net/highcharts/cqjvD/1261/
I got a bug similar to this one but after investigation boost module turned out to be the culprit.
Whenever your data exceeds boostThreshold, csv and excel exports return empty data sets. Internally getDataRows seems to return only two rows then. It happens both in my project and your jsfiddle: http://jsfiddle.net/highcharts/cqjvD/1261/