What is the true gender-neutral equivalent of "man"?

"The human being is a social animal."

(See relevant Google Ngram Viewer chart that confirms that this usage exists and seems to be more common than something like "The human is a social animal.")

"Man" in this construction just doesn't behave like a normal singular noun, so you can't substitute one. You have to use an ordinary singular noun phrase, and singular noun phrases in English usually have to include some determiner like the or a (the main exception I can think of is mass nouns like "water" or "grass"). In these kind of "generic singular" contexts, the definite article the is often used as the determiner of the singular noun.

It doesn't seem very unusual to me that the word "man" developed unique grammatical features like this. It has a very general meaning and words like this often undergo some degree of "grammaticalization". In German, "man" has become even more grammaticalized and is an indefinite pronoun. In French, the Latin noun homo "man" developed into the indefinite pronoun on, which subsequently gained a further grammatical use as a first-person plural pronoun.

People are social animals

College students* are social animals (*or any other labeled group bringing to mind people within a context)

I think the problem with "human" is that it is too synonymous with the scientific classification and in doing so misses emphasizing the dichotomy of culture vs biology.

Culturally Formed Outcome vs Biological Underpinning

That is why I bring up "college students". A college student's life is defined by weeks a semester, tasks relating to studies, walking to lectures, writing papers, and we could throw in stereotypes about drinking coffee and beer, a most commonly unwed child-free state etc.

'People' is the largest "group" of living, socialized human beings, I can think of.

... but it still doesn't convey "man" , because "man" meant something more that reflected ideas of the times within which it was used

"Man" , as it was used, I believe, implied heavy doses of "use of tools", and "artistic expression", and "technological achievement" and "the march of history". The word represented a "world-view" different than "people" means today.

When we've put aside the word "man" we've also put aside human-centric notions that ... gee, we can even talk about the changes in perception of our place in the universe once we knew the Sun didn't revolve around the Earth and that stars were millions of clumps of hydrogen fusion spun across a universe of multiple galaxies.

I'm not sure it is even possible to mean "man" the way it was meant before because what we know now prevents the word from evoking what it meant then.