Nature of Enlightenment How did different people with different philosophies and ideologies attain the same enlightenment or did they not? Buddha attained enlightenment and so did Adi Shankaracharya, but their philosophies are different (not to mention ramanujacharya, nagarjuna and many others). Or is it the case that we can never really know who attains enlightenment? Well I know that there is similarity between Buddhism and Advaita Vedanta but at the core they are very different. How did they attain the same enlightenment knowing the real truth in two different ways? At least one of them is wrong.
Well some say the truth can be interpreted in different ways but if advaita says self exists and buddhism denies it there is a big problem you can't make them compatible with one another.
You're absolutely right that they are only superficially appearing similar, but when you dive deep, they are completely different.
This has been discussed in numerous answers, which I will not repeat here - this answer, this answer, this answer, this answer, this answer.
The original teachings of the Buddha are systematic (the four noble truths), empirical (the three marks of existence), soteriological (the Noble Eightfold Path) and avoids intellectual gymnastics and obscure mysticism (Parable of the Poisoned Arrow, Acintita Sutta, Sabba Sutta), while Advaita is metaphysical and based on logic, philosophy and intellectual gymnastics.
For e.g. the Eternal Self as the Universal Consciousness or Cosmic Consciousness that is standalone, indivisible, eternal and witnesses through all living beings is a very elegant idea in Advaita Vedanta.
But the Buddha's empirical observation and realization is that consciousness is dependently originated on the six sensory media, and it is conditioned and impermanent (see MN 38). Even the mental idea of the self is also dependently originated, conditioned and impermanent. This can be realized generally through the Noble Eightfold Path, and specifically through vipassana.
So, which is true? If you dive deeper, you can see for yourself which is true. One is a very elegant intellectual model, while the other is empirically verifiable.