On the scale of clickbait headlines this is pretty minor.
Riemann hypothesis is an important problem that has gathered a lot of attention over an extended period of time. Is it the biggest? Whose to say. It is at least up there in the list of major open problems. Its not like its an obscure problem by any means.
In my mind, the justification for referring to RH as the “biggest” open problem in mathematics is its importance in numerous fields of mathematics. It’s hard to take three steps in analytic number theory without running into Riemann. Zeta functions — and with them RH — have transcended number theory and are fundamental objects of study in other mathematical fields, most prominently representation theory and the Langlands Program.
> The Riemann hypothesis is noteworthy for its appearance on the list of Hilbert problems, Smale's list, the list of Millennium Prize Problems, and even the Weil conjectures, in its geometric guise. Although it has been attacked by major mathematicians of our day, many experts believe that it will still be part of unsolved problems lists for many centuries. Hilbert himself declared: "If I were to awaken after having slept for a thousand years, my first question would be: Has the Riemann hypothesis been proved?"
Hilbert posed several nifty problems over 100 years ago, including pithy cool ones not unlike Riemann's zeroes.
https://en.wikipedia.org/wiki/Hilbert%27s_problems
[ edited for my own typos ]