The manga has been inconsistent when it comes to things that exists or not during the timeline. At first it was consistent since the timeline was only a few years different from when it was drawn. However later when the series has passed +30 years and the timeline isn't that consistent, then such as this would happen.
The mangas current timeline is near the beginning of the 2000's, however we have seen instances of smartphones and lastly the One World trade center. What is more inconsistent is that the Twin Towers was actually shown a bit earlier during the last 100-200 chapters, so it is most likely due to an incorrect reference by Morikawa/his assistants looking at the current Manhattan, instead of the one from the early 2000's.