I've done a bit more digging. I didn't find Freud's essay, but I found this paper by Tom Loveday on it:
https://www.academia.edu/22896901/Science_and_Art_3_The_Antithetical_Meaning_of_Primal_Words
Apparently, Freud was referring to the work of philologist Karl Abel,
Über den Gegensinn der Urworte, (1884), which Google translate thinks means 'About the Antithesis of the Original Words,' but I could see how someone gets 'primal' out of that.
...and to make it even more involved, that article I found has Loveday quoting Freud quoting Abel quoting yet
another guy, Alexander Bain, who said: "The essential relativity of all knowledge, thought or consciousness cannot but show itself in language. If everything that we know can be viewed as a transition from something to something else, every experience must have two sides; and either every name must have a doubled meaning, or else for every meaning there must be two names."
Freud explains the philology (which I assume he got from Abel) as:
‘A word that originally bore two meanings separates in the later language into two words with single meanings, in a process whereby each of the two opposed meanings takes over a particular phonetic “reduction” (modification) of the original root.’
Freud, of course, uses this to explain both dreams and what are now called Freudian slips:
"It is plausible to suppose, too, that the original antithetical meaning of words exhibits the ready-made mechanism which is exploited for various purposes by slips of the tongue that result in the opposite being said (of what is consciously intended. (fn to final sentence.))"
....in other words, some 19th century German philologists (or at least Abel) thought that Egyptian contained a lot of words that mean two things at once (typically opposites), and labeled this a characteristic of 'primal' language, with the later languages refining further to separate those ideas into different words with the same roots. The implication is, then, that a 'modern' word with two opposite meanings is a 'left over' primal word that wasn't forced into only one side of its meaning.
I certainly have no way of evaluating the validity of any of that, other than to acknowledge that languages are based on roots and some words do have two meanings (and even opposite), like terrible, awesome, or cleave. But if "cleave" is really the go-to here, I guess not many words fit this description, and I'm not sure how 'primal' a word like 'cleave' is. I know one of the reasons we keep the meaning 'cling to' with cleave is because it appears in English translations of the book of Genesis: "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." Genesis 2:24 King James Version, which Jesus quotes in Matthew 19:5 and Mark 10:7. I'm not sure I've heard cleave (with that meaning) outside the context of husband and wife; obviously Arwen uses it of Aragorn in LotR. But I think it is noticeably archaic, and that 'standard' usage is the other meaning - to split asunder, like a meat cleaver.
But at the very least it is clear that your prof didn't make this up, and that it really is called that!