It is like maths for existentialism.
People might deny or not notice it but that means that people ignore and deny the truth and not make it false.
Anyone mentally capable can deduce the same things and achieve nirvana without ever knowing about buddha.

One of my life quests was to make my default state uncaring to the extreme. In order to be untouchable by circumstances and failures no matter how bad things get until everything became tasteless. You can't be sad if you don't differentiate between the things you like and things you don't like. But you can't be happy either. All reality becomes tasteless and you can't bother to struggle for either outcome, you let reality do whatever it is meant to do because you don't need anything anymore. You aren't attached to things anymore. If any outcome is equal then you can't attach to hopes, will and wishes too because they are equal to nightmares and failures. Detachment. I call it the first major development on person's path.

Anyone living for long enough will realize it by the sheer amount of experience and looking at patterns. People might not trust word but they would trust their own eyes when reflecting on their past and what they saw. The more honest and selfless you are in seeking for patterns the faster realization comes and I don't mean it is only detachment as in 4 noble truths but I can't share it. It goes exactly the same case as for learning maths, if you honestly try and look what connects things, patterns, it accelerates because you are not ignorant and actively seek answers, separate correct from incorrect.

But it is not the finish point when you detach and realize it is futile to cling to anything. There is so many derivatives of the realization, some to be understood by people freshly after realization and some still unknown by people 50 years after realization. There is a major breaking point that shatters detached people once again but this time it sends them on a one way straight path only closer to nirvana which people rarely achieve anyway. That is the second major development.

It is a mistake to think that you are superior after you genuinely realize the nature of reality at the first development. It only means you are now able to get on the path to perfection and not perfect yet, not even yet on the one way path to perfection, just able to advance to it as it is a prerequisite.

I could elaborate on the second major development but I don't think it would be beneficial. If you stop seeking you might fall to ignorance. You don't have to push for it, or read every buddhist book, listen to every monk and every kind of understanding. But instead don't settle forever in ignorance and advance in your pace. A person could reach their point of will to learn and discover, or their limit of perception or any other factor. All are fine.

But there are much deeper truths at the last development which is everyone's personal contact with ultimate reality. A person might not grasp the final truth, but only ever touch it, or worse: believe it. It is the point where you choose to either to never advance further and stay impure, or go beyond it and hop on the path directly to nirvana which would be the third and last development in my opinion. That point goes as nuts as 0/0 in maths.

Picture is unrelated but I like it.