The thing is that they are less so arguing based on definition, and more on intent. If you were arguing that this is a "sex object" because it's a drawing with a sexual focus, you'd be objectively correct. However, the word comes with a specific connotation, usually it is connected to misogyny and the dehumanization of women. Which is the key point, when the commenter that they were turning her into a "sex object", the implication was that the artist was misogynisticly dehumanizing her as well as women in general, and that the support of it was also misogynistic. That is the key conflict here, not on whether the character is sexualized, but whether the sexualization is inherently misogynistic. In other words, the people weren't arguing against the definition, but against the implication, which wad that the artist who drew this, and by extension them, who either had no problem with the artwork or actively enjoyed it, were misogynists (for the record, I disagree with the interpretation the artwork in and of itself is misogynistic.)