@shivsomashekhar
Interesting.
The Upanishads on the other hand are non-dualistic, is it not?
No.
1. Without an Advaita commentary, you will be hard pressed to find Upanishads that claim the Universe is an illusion or the two-level truths or that Jivas lose their identities (effect of merging) on Moksha. If the Upanishads are Advaitic, you should see several such statements. But you never find such statements* in Upanishads or the Sutras or the Gita. You only find them in Advaita texts. And the two level truths come from Mahayana Buddhism.
2. Advaita has to interpret the various dualistic Upanishads to mean something other than what they say
3. All non-dualistic Upanishads are accepted by dualistic philosophies too and they interpret them in a manner consistent with their doctrines.
On the other hand, if you read the Upanishads (and the Gita) from a oneness-difference perspective, you need very little interpretation. Jivas are real, the world is real, Jivas can find Moksha, their identities persist after Moksha and they are always fully dependent on Ishwara. Like Sunlight and the Sun...and hence, oneness-difference.
There are many Upanishadic verses that says you and brahman are one and the same and that there's no difference whatsoever (exactly what advaita preaches).
Every one of those statements is equally accepted by Dvaita and Vishishtadvaita too - consistent with their doctrines. Therefore, these are not exclusive to Advaita.
*The last couple of Brhadaranyka verses are an exception which sound very Buddhist.
Two truths doctrine - Wikipedia