Insan has been understood to refer to all human beings, or only Adam, but it does not change the meaning significantly. It seems most appropriate to understand it as referring to all human beings.

This ayah is understood to be a rhetorical question, along the lines of, "Was there ever any time when man was such an insignificant thing that he was not worthy of being mentioned?"

Here are 3 ways it has been understood:

(a) This could be after the process of creation of the human being had begun but before human beings had populated the earth.

This idea is supported by a narration from Imam al-Sadiq (A) that it refers to 'when he was created, but unworthy of mention'.

It is also narrated from Imam al-Baqir (A) that this verse means that 'the human being was mentioned in Allah's knowledge, but not in His creation'. That is, the human being had been decreed to exist, but did not exist yet on earth.

(b) Some exegetes also say this could be before the human was created at all. (The human only existed in the divine decree and knowledge, so the human was "mentioned" by God but not by anyone else.)

(c) It has also been understood to mean, "Was there ever any time when God was ignorant of the human being?" The answer to that is no, since God had knowledge of what He would create.

All of the meanings can work together too since the point is to emphasize the smallness of the human being compared to the divine, which is a theme that continues in the next ayah.