It's not the first time that he implies that, nor the last time that he will, or even a novelty for him to talk about themes that aren't exactly in his games (USgamer:
Does Detroit Become Human's David Cage Actually Know What His Game is About?).
It's not that Detroit didn't talk about that (or did it well or not), it's more the implication that it had never been done before that makes it ridiculous.
An easy reply to that question (could video games talk about those topics?) that has already been done to death is of course Nier Automata, but talking about segregation and humanism, doing parallels (hamfisted or not) with our society, the right to be different... has already been done in other video games (including previous games in the Nier/Drakengard franchise). The Deus Ex and Bioshock franchises are also big examples for some of those themes, same thing for the Shin Megami Tensei franchise.
These themes are not only recurring in Japanese science fiction/fantasy games, but also in multiple classic western RPGs. What are some of your favorite games talking about "such serious things"?
edit:
by popular demand (or incomprehension), here's this other tweet to clarify Mr Cage's thoughts on the matter:
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