Are you, by any chance, reading the subtitle as “The greatest mathematician, who never lived” (i.e. Bourbaki was the greatest mathematician, and he never lived) rather than as (what's written) “The greatest mathematician who never lived” (i.e. among the mathematicians who never lived, Bourbaki was the greatest)?
The latter is a common snowclone especially in headlines, like (from https://www.google.com/search?q=%22the+*+*+you+haven%27t+hea...) “The Most Important Filmmaker You Haven't Heard Of”, “The Coolest Apps You Haven't Heard Of”, “Meet Vulfpeck, the best band you haven't heard of yet” — e.g. the last one doesn't mean that Vulfpeck is the best band, just that among bands you (probably) haven't heard of, it's (according to the author) the best.
The latter is a common snowclone especially in headlines, like (from https://www.google.com/search?q=%22the+*+*+you+haven%27t+hea...) “The Most Important Filmmaker You Haven't Heard Of”, “The Coolest Apps You Haven't Heard Of”, “Meet Vulfpeck, the best band you haven't heard of yet” — e.g. the last one doesn't mean that Vulfpeck is the best band, just that among bands you (probably) haven't heard of, it's (according to the author) the best.