A statement of faith attributed to Athanasius (296-373 AD) begins with the following excerpt:
Athanasius - Statement of Faith
We believe in one Unbegotten God, Father Almighty, maker of all things both visible and invisible, that has His being from Himself. And in one Only-begotten Word, Wisdom, Son, begotten of the Father without beginning and eternally; word not pronounced nor mental, nor an effluence of the Perfect, nor a dividing of the impassible Essence, nor an issue; but absolutely perfect Son, living and powerful (Hebrews 4:12), the true Image of the Father, equal in honour and glory. For this, he says, 'is the will of the Father, that as they honour the Father, so they may honour the Son also' (John 5:23): very God of very God, as John says in his general Epistles, 'And we are in Him that is true, even in His Son Jesus Christ: this is the true God and everlasting life' (1 John 5:20): Almighty of Almighty. For all things which the Father rules and sways, the Son rules and sways likewise: wholly from the Whole, being like the Father as the Lord says, 'he that has seen Me has seen the Father' (John 14:9). But He was begotten ineffably and incomprehensibly, for 'who shall declare his generation?' (Isaiah 53:8), in other words, no one can.
Leaving aside Athanasius's contentious use of John 5:23 (covered here) and 1 John 5:20 (covered here), notice that he uses Isaiah 53:8 to support the doctrine of eternal generation, and specifically, that Jesus was "begotten ineffably and incomprehensibly".
The verse in question says:
Isaiah 53:8
8 He was taken from prison and from judgment: and who shall declare his generation? for he was cut off out of the land of the living: for the transgression of my people was he stricken.
Translated as above in the KJV, the reference to no one being able to declare His "generation", also translated "descendants" (ISV, NLT), would apparently refer to the fact that Jesus had no physical descendants - "for He was cut off out of the land of the living".
Others render the passage as something like, "Who could have imagined his future? For he was cut off from the land of the living, stricken for the transgression of my people" (NRSVUE, NABRE). Here, "future", also rendered "fate" (CSB, CEB, HCSB), would be referring to the fact that He was "he was cut off from the land of the living, stricken for the transgression of my people". In other words, "Who could imagine how His life would end?", and then it states that it was tragically cut short, answering the question.
Additionally, others translate the passage to say something like, "and as for his generation, who considered that he was cut off out of the land of the living, stricken for the transgression of my people" (ESV, NIV, NASB). Here, "his generation" would apparently refer to His contemporaries, who did not understand that He was being stricken for their transgressions, or did not care about His death.
The assertion that this refers to an incomprehensible eternal generation would not make sense in context. If the first part of the statement was saying something like, "no one can describe His incomprehensible eternal generation from the Father", it would have nothing to do with Him being cut out of the land of the living. Yet, the phrases are connected by the Hebrew word kî (translated "for" in the KJV). That is why the ESV combines them into a single thought. The KJV makes it a rhetorical question. In either case, they are directly connected, and a statement about eternal generation does not fit into that context.