I mean in the end it's probably just preference of the author and where you place the priority.
Like both terms could be used to describe a person with some level of abstraction and distance. Though "human being" quite literally focuses on the fact that it is a being and that it is human, while "individual" focuses more on a property of that person. Individual apparently stems from indivisible but apparently the meaning shifted from the atomistic sense of that word to the consequence of being an atom, namely that it describes an entity that is distinct from others.
So if you look at a particular human being, you inevitably also look at an individual, but not every individual needs to be a human. A grain in sack of rice is as much of an individual as a human being, yet it's not really as much of a human or a being.
So of course if you place the focus on human improvement not for the sake of serving a purpose but for it's own sake (as was done by humanities and their focus on literature, poetry and other "useless" endeavors) then you have to deal with the individual qualities of a person and consider them as individuals.
Though you should distinguish between "the individual" and "individualism". The individual and it's individuality is about them being a distinct entity and their distinctive features, while "individualism" is a a framework that put the individual at it's focus, that is it analyzes it's environment from the perspective of an individual observer. Unlike for example collectivism that places the focus on the collective as a whole and considers the individual as just a part of something bigger. Like for example this bible quote "Now you are the body of Christ, and each one of you is a part of it.". Contrary to that individualism would argue that the individual already is a whole in itself and looks at the world through that sense.
So it's not that the enlightenment invented the individual, but rather that it placed a larger focus on the individual. Rather than treating other people as a means to an end or as disposable parts of a larger entity they were put in the center and seen as ends in itself. And so you had this antagonism between the collective and the individual. From which developed more rules protecting the individual from the overreach of other individuals or collectives.
That is, in practice, you could be an "individual" without being a "human being" in the sense of the Renaissance Humanism, although it was the Renaissance Humanism which made possible the Enlightenment individualism.
I'd say it's the other way around being a human being was enough to be accepted as an individual. Not sure if the humanitarians expelled people from being humans if they didn't fit a criteria.
That would explain why the socialism and classical liberalism which were born out of the Enlightenment can make use of the same term "individual" or "citizen" (and with it the concept of "freedom") while completely disagreeing about its nature (socialists would say that individuals must help each other and depend on each other (freedom is believed to exist if and only if the basic material needs are fulfilled), while classical liberals would say that individuals must be independent of each other (freedom is orthogonal to the issue of the fulfillment of basic material needs, and refers to some civil rights to be protected legally (property right mainly)).
"citizen" just means resident of a city and individual just means "distinct entity". So these terms are so broad and inoffensive that, as terms, anybody could use them.
With respect to freedom, socialists and classical liberals you mix up a whole bunch of stuff that you should better wrap in a different question.
Just as a sketch. Freedom is usually a combination of "agency" (freedom to do something) and a lack of coercion (freedom from something). And on top of that you have the problem that freedom finds it's limitation in the freedom of other people because if it doesn't, it becomes coercive or limits their agency. So "freedom" is a much more complicated mess that you have to constantly negotiate with other people, while classical liberals thought it's enough to encode a set of rules and privileges of the individual and the rest will follow from that.
One glaring flaw is as you mentioned that if your basic needs are not met you don't have freedom. There is no agency in your doing because you are driven by fulfillment of your needs or you die. That's not really an issue for these classical liberals who were largely rather rich, often slave owners or colonialists and who were more concerned with property. But which is a huge issue for all those without property and in the struggle of sustaining their existence.
So while classical liberals often took on an individualism to the point of egoism and egocentrism, socialism was more geared towards the forming a mutual collective and unfortunately more prominent implementations a collective that is above the individual and not made up mutually from individuals. Though there's a a lot of room between individualism and collectivism to struggle about what matters more.
