Suppose someone did not take a diploma/degree, can he get hired with a non-traditional education path with online certificates from courses online (such as Coursera or Udemy)?

I believe so, but you definitely have to showcase your skillsets in other ways, such as past work/internship experiences, side gigs, personal projects etc.

Yes, especially if you display sufficient skill and experience in an industry that does not put a huge emphasis on degrees.

That said, it is unlikely that the public sector hires candidates who have no diploma/degree.

Additionally, online courses are not a great differentiator as well. Anyone can do it, and additionally, it can be done by others.

Which brings me to my main point:

You need to know what differentiates you from the rest of the candidates applying to the same job as you. What do you offer that the other candidates don't have, such that the employer is able to overlook the fact that you don't have a diploma/degree?

In the current working world, a degree/diploma is hardly a differentiator, because most have it. This is unlike our parent's generation, where going to university is already a big differentiator.