https://en.wikipedia.org/wiki/Occam%27s_razor
Why is it that conspiracy theories seem to always require a more complex answer than the truth?
One thing they always fall down is the idea that government bureaucracy is so quick and competent that they could do something like installing mass surveillance at a moments notice while the country is in chaos without it leaking due the next 20 years
It didn't have to be installed, it was already, although in a pre-Snowden era, governments doing Internet surveillance was the conspiracy theory. My speculation is that more international traffic got routed through the existing surveillance infrastructure, causing a bottleneck that slowly got better.
My point is, the Internet rerouting around damage doesn't explain the slowdown, mostly because it happened days after, not immediately, meaning the damaged infrastructure was not critical for the global internet.
The attacks unleashed a huge intelligence operation, and it's reasonable to expect a parallel intelligence operation online. If the latter caused a traffic bottleneck or not, we can't know because it was never addressed, it's speculation on my part.