This is very possible: in the first novel, Ace explains, “Here, we call everybody human–merfolk, beast people, and people like us, too.”
Ace stretched his arm out toward the bed and wrote “beast people” on the sheets. “You don’t have ’em in your world, Yu?”
“No.” He shook his head. “Only humans go to school and talk and stuff.”
“Humans.” Ace frowned. “So then do you have merfolk?”
“Merfolk?!” Yuya gaped at him. “You mean, outside of stories?”
“Stories!” Ace laughed. “Your world really is weird, Yu. Here, we call everybody human-merfolk, beast people, and people like us, too.”
The novel and the game can vary so it is best to not think of something being canon in one meaning that it is canon to them both, but there does not seem to be anything in the game that refutes this point (as of this writing).
Sebek referring to Azul as a human is a point in Malleus’ Glorious Masquerade vignette: Azul compliments Malleus, so Sebek corrects himself to call Azul by name instead (even referring to him with the “-senpai” honorific) after he calls him human.
Sebek also calls Ruggie human during New Year’s (removed from his EN dialogue), and calls Leona human during Malleus’ dorm vignette.
It is an interesting point that none of mermaid/beast-person characters ever correct him.
There is a scene during Spectral Soiree where a possessed Ace calls Floyd and Leona human. Floyd responds with “I know you are, but what am I?,” as though insisting that he himself is not human, but this was a joke that was added to Floyd’s EN dialogue.
In Floyd’s actual dialogue in the original game he responds, “You’re human, too,” as though he considers himself to be as human as Ace.
Returning to Sebek: he refers to both Jack and Ortho by their names during Fairy Gala IF, though he does address the group of the Prefect, Grim, Ace, Deuce, Epel, Jack and Ortho as “humans” collectively.
A part of TwstFes 2023 was a live reading of two scenes by all the voice actors, in character. Sebek and Ortho’s VAs were in one of the two scenes together and Sebek’s dialogue included him referring to Ortho, directly, as “human,” but this may not be considered canon to the game considering the circumstances.
While Sebek is the most infamous of the cast for his use of the word “human” he is not the only one, with both Malleus and Lilia also referring to non-fae as humans.
In Malleus’ dorm vignette we have both characters referring to “humans and fae” as though those are the only two variations in this universe, which may add credit to the “everyone who is not fae is human” theory.
This very topic being was discussed in a Twstsoku thread in 2023 where a similar conclusion was reached. Someone in the comments asks if beast-people are really considered human in the game despite how they are canonically the descendants of animals and they receive the response, “humans are animals.”
Twisted Wonderland Reference Note 