In short, he is suggesting that these words were written as an editorial comment by Moshe, as opposed to something that Adam said at the time. Here's a rough translation:
Therefore a man... "Al Ken" ("therefore") refers only to the past and the present, and "Lachen" refers to the future. Therefore, these are not the words of Adam, but the words of Moshe. This is in addition to the fact that Adam did not yet know of (the concept of?) mother and father.
As a support to my understanding, Daniel A Klein (here) translates it as follows:
Therefore it is (al ken) that one leaves – The term al ken always refers to past or present conditions, while the term lakhen [also translated "therefore"] refers to the future. Thus, these are not Adam's words but those of Moses, aside from the fact that Adam did not yet know the words "father" and "mother".