Admiral Woofington
Member
I guess it depends on what you consider a harem. If you consider a cast which is disproportionately female a harem, then it's a harem.
As someone who has read the manga I can assure people that at least as of right now it has harem-like qualities but it's not a harem. A harem, at least in my honest opinion, implies that every single female character wants to be with him and the show will largely focus on him having scenes with at least one of the female characters every once and again and/or involves them fighting over him. It focuses on the relationships of the females with the mc.
It has harem-like qualities in the sense that yes there are a couple of women attracted to him but it hasn't focused whatsoever on that aspect and instead focuses on Gary Stu Batman being unstoppable in science and war with the occasional incest-y subtext. I know it's hard to tell people to ignore the incesty stuff when I'm such a huge advocate of anti-pandering. But onii-sama's gary stu levels are hilariously addictive.