You are completely entitled to your own opinion, but personally I believe that the 2001 attacks accelerated the divide to the point that it may have made the difference between Trump getting elected or not. Without the Iraq war a whole pile of stuff that laid the ground for Trump would not have happened.
We agree that this was already happening prior to 2001, but I simply doubt it would have resulted in the extreme situation that we have today. Social media, similarly is an accelerator, not a cause.
The increased polarization in the United States is a product of the end in the mid-1990s of the overlapping pair of partisan realignments starting just before WWII and a return to the normal condition encouraged by the US electoral structure of a strong, stable, sharp partisan divide, combined with modern mass media technology.
The 2001 terror attacks are coincidental, not a cause.
Denying this is closing your eyes to major structural problems with the political system in the United States and, as long as you deny something, you are categorically unable to fix it.