I suspect there's an element of "cause and effect" here. I would expect degree-holders to have a better pass-rate than others, but not because they have a degree! The most important thing is having the dedication and the ability to study. Degree-holders have already proven that they've got this ability, because it's required to get a degree. That doesn't mean that people without a degree aren't capable of studying and passing all the ATPL exams first time - it just means that this ability hasn't already been proven.
FFF
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(PS - I have a degree, and I passed all of the Module 1 exams first time, with good marks. Haven't done the Module 2 exams yet. Don't believe my degree is at all relevant. The fact that I study in every spare minute is. Didn't vote on the original pole because there wasn't an appropriate option.)