Amateur comes from the Latin amar which means to love. An amateur is someone who does something for pleasure rather than money. Sometimes people use it to mean a novice or someone who is unskilled but in this case the original meaning applies.

I think amateur in this case refers to non-professional / research mathematician with a PhD working at a University or a Research Institute. You still need to have some serious math chops to get this kind of result, it just isn't reflected in your official title or degree.

IIRC this guy or his associates dug up soil from a medieval burial site near Cambridge (England) to try and find a bacterial enzyme capable of breaking down 7-ketocholesterol (and hence potentially to cure artherosclerosis). Maybe not Indiana Jones but certainly ingenius and intrepid!