I mean, it just seems a little presumptuous to state with certainty who is at fault without knowing what's causing the problem.

This is kind of the issue with buying a condo with shared spaces like attics and crawl spaces and utility demarcs; there could be any number of people in those areas doing any number of things. If it turns out the problem is outside at the MDU then there will be no charge to you as that is Comcast's responsibility to maintain. If the wiring is faulty inside the building somewhere, that could cause a charge to you. That kind of damage could be caused by many different things, a lot of which are hard for you to control in a multi dwelling building. So, yes, you could be charged for something that someone else was responsible for causing (no way to know if that someone was a Comcast tech or not at this point) but that's almost seems like an inherent risk of living in a condo building. If it turns out that the problem is in the building and you're able to determine that it was the fault of a Comcast tech, I would definitely bring that up in the /r/xfinity_comcast sub and see about being credited for the tech visit.