I get it, not all the manga is read by the staff, so someone most likely reported it but come on.
That's exactly it, though. They can't reasonably know what is and isn't a spoiler 100% of the time so sometimes they're going to have to assume someone reported one correctly. When I was edited for a spoiler once and it wasn't one, I shot the mod who did it a
private message about it, in which I politely pointed out what happened and why my post wasn't a spoiler, and they ended up going back to fix it after apologizing to me.
Calling them out in the thread itself and taking an attitude ("Are they blind??") over it is probably not going to move anyone to do anything though.