Not the parent poster, but I do think it's worth putting a finger on what might simply be a cultural gap between modern (and especially fairly plugged-in) Americans and others who either genuinely have not been socialised into the present-day US discourse about race or are deliberately acting defiant about it due to how it conflicts with the framework that they grew up with.
It seems to me that according to present-day American norms, including a remark such as the one that you did is simply considered good form; you show that even if you are not actively working on the great political causes that are being so eloquently argued to be of existential importance by their protagonists every day, you are certainly aware of them, support the good guys and ready to do your part for their cause if it you are in a situation in which you can contribute. Meanwhile, for those who adhere to a framework in which the prescription for making sure historical instances of racism don't repeat is to banish race as a category from human thinking and discourse as far as possible, the very mention of it in a context where there seemed to be no external reason to is gauche in a similar way to if you suddenly started talking about sexual prowess (taking care to insert a parenthetical remark about what you believe about your own). It doesn't seem to me like your explanation of the "why" here is particularly helpful to cross that gap of expectations, either; it only really makes sense if the reader is already primed to assume that describing people as "interesting characters" and unusual people must at least suggest something about race and gender, but from the other side's perspective this is not any less gauche, especially as it might be taken to suggest that what makes those characters "interesting" to you is just a matter of those characteristics.
Even worse, if the culturally remote reader's degree of familiarity with modern US norms is not zero, the one blurb they likely have in mind is something like "the US has a self-admitted racism problem". In that case, a remark such as the one you made might not just register as introducing a topic that is unsuitable for polite conversation, but actually as betraying attitudes that are opposite to what you wished to communicate - imagine if for instance a speaker of the German police, talking about something mundane such as their strategies for fighting pickpockets in Frankfurt, injected a line lamenting the small number of Jews in the force. Indeed, if my read of US society is correct, antisemitism is one context in which the other (older?) version of antiracism norms still survives in it. To get a rough model of how a given post or statement would sound to the cultural outsider, then, you could try to imagine your post with everything mentioning race replaced with analogous remarks along the "Jewish or not Jewish?" axis.