I worked at SpaceX at the time, and I cannot speak for the company, but I can tell you that approximately nobody inside SpaceX took the idea of a sniper seriously. There was a lot of internet talk about it, and it was one of hundreds of avenues that were explored, and ruled out basically as soon as it was explored.

The very interesting part of the liquid oxygen failure (and this was published in the investigative findings) was that the liquid oxygen that became trapped in the fibers was actually cooled and compressed into solid oxygen - you can read some details here: https://www.americaspace.com/2017/01/02/spacex-closes-amos-6...