First of all, the post technically doesn't claim that formulation is humanistic. It only claims it's more humanistic than 'the other', which you didn't quote, but here it is:
Act only according to that maxim whereby you can at the same time will that it should become a universal law. (Grounding for the Metaphysics of Morals (link is external), p. 421)
Furthermore, if you read the context, this post not only claims the other formulation is more humanistic, it also claims it's less formalistic. Which means:
formalism {noun} = a style or method in art, music, literature, science, etc. that pays more attention to the rules and the correct arrangement and appearance of things than to inner meaning and feelings
Now it becomes more clear what the author means: the first formulation (the one I quoted) can easily be used in some kind of (perfect) world model: it's very technical and focuses on logical soundness. The second formulation however is geared more toward 'daily use' and gives a somewhat more practical idea, something we can actually try to use.
The word 'humanistic' is not used in the strict sense of the word you looked up here. What is meant is more 'human-oriented' or something like this. This formulation of the Categorical Imperative is actually called the Humanity Formula:
This formulation states that we should never act in such a way that we treat Humanity, whether in ourselves or in others, as a means only but always as an end in itself. This is often seen as introducing the idea of “respect” for persons, for whatever it is that is essential to our Humanity. Kant was clearly right that this and the other formulations bring the CI ‘closer to intuition’ than the Universal Law formula. Intuitively, there seems something wrong with treating human beings as mere instruments with no value beyond this.
Example
Using the first formulation we can argue we shouldn't lie:
If lying would be a universal law, everyone would lie. But then everyone knows everyone is lying, and thus there's no point in lying anymore, because lying assumes the other person thinks you're speaking the truth. Therefore, we shouldn't lie.
But proofs like this aren't intuitive for most people. The other formulation gives a much more intuitive proof (and to have an easy proof I'll not take into account lying for the other's sake):
If I lie to someone, I don't take into account the negative influence this will have on him. I would then not treat him as an end in itself, but only as a means. Therefore, I should not lie.