No, he's not an amateur but a retired statistian. And the fact that LaTeX is the standard for DIY typesetting in academia nowadays has nothing to do with your competence as a mathematician, or the rigour of a proof published as a Word document. Likewise, it doesn't matter if you work with pen and paper or with pencil and paper.

This title is absurd.

Well it got me to click on it. Which in the end I’m pretty happy about; it was a very interesting article. Can clickbait be justifiable if the end product is good?

You're opening a can of worms there.

Which is pretty thin. I wouldn't consider mathematicians of the pre-word-processor era amateurs.

Not just that, but that he did it with rudimentary tools.

'Amateur' would seem to conflict with 'retiree.'

Not really. You can be an expert in one field and an amateur in another.