The statement, only Brahman is real (sat) and everything else is unreal (asat), is entirely wrong. Advaita views the world as mithya and not asat. Please see my previous answer.
There are three methods of approach in Advaita vada for creation of the world
(1) The ajatavada is represented by no loss, no creation, no one
bound, no sadhaka, no one desirous of liberation, no liberation. This
is the Supreme Truth. (Mandukya Karika, II - 32).
(2) Drishti Srishtivada :- Simultaneous creation.
(3) Srishti Drishtivada is plain (Gradual creation and knowledge of
it).
Now to the mithya of the world, Talk 315, Talks with Ramana Maharshi
The dream water quenches dream thirst. The dream creation is however
contradicted in the waking state. The waking creation is contradicted
in the other two states. What is not continuous cannot be real. If
real, the thing must ever be real - and not real for a short time and
unreal at other times. So it is with magical creations. They appear
real and are yet illusory. Similarly the universe cannot be real of
itself - that is to say, apart from the underlying Reality [Brahman].
I do not know how the author classifies the upanishads as old and new. For vedanta followers, the upanishads are eternal, authorless and beyond time. There is no classification of old and new.
While some upanishads discuss creation of the world, some upanishads such as Mandukya upanishad talk of mithya of the world. However, the world is not unreal but only mithya. That is the world, can not exist apart from the underlying Brahman but appears differently. It appears as a snake which is unreal but the rope is real.
Even if you take the above statements, you can read Shankara Bhashya for the same. Let us take the example of BU II.1.20 and read the bhashya here in page 290 onwards. The bhashya runs for around 30 pages and ends at 320. Shankara clearly establishes in the bhashya that the Brahman is the substratum of the world. He cites other upanishads, MU, Isa and even the Bhagavad Gita in support of the argument. Shankara states that Tiny sparks etc. in the text signify oneness with fire. For example, to quote his bhashya
For the passages are meant to convey the idea of oneness. We notice in
life that sparks of fire may be considered identical with fire.
Similarly a part may be considered identical with the whole. Such
being the case, words signifying a modification or part of the Supreme
Self, as applied to the individual self, are meant to convey its
identity with It. That this is so appears also from the introduction
and conclusion. In all the Upanisads first identity is broached, then
by means of illustrations and reasons the universe is shown to be a
modification or part or the like of the Supreme Self, and the
conclusion again brings out the identity.
'This all is the Self' (II. iv. 6), then arguments for origin of universe is presented, then it is shown that that the creation is nothing but a projection of Brahman and then it will conclude that the self is Brahman (II.v.19).
