I don't play an insurance agent but ... when I asked about anything I need for coverage with our agent - I asked about any and ALL endorsements that one could get. She mentioned the sewer back-up. I almost laughed and said "isn't that my problem and I just call the plumber?". She proceeded to explain that sewer back-up is not their problem when the city/county is at fault. I asked "are they just going to start pumping this gross stuff back to me or something?" she giggled but said that in some weather situations the problem with water coming into a house could be due to the fact that all the sewer drains are clogged or filled beyond capacity and has literally no other place to go - hence that's why it is coming into your house from the ground up - not considered flooding but then would be considered sewer back-up. She stated that the city/county is not responsible for your that as it would be considered an "act of God" in their policy and each and every homeowner is responsible for their own property.
Again - lots of crazy "insurance terminology" but that's their game. I just try to keep myself & home covered. Heck - we even have an earthquake endorsement and we are nowhere near the fault lines but about a year or two ago a very small earthquake hit in Walhalla, SC which is couple hours away - so we got it.