I can see why you are miffed with the comments, but I'd take some comfort over the fact they never changed your main battery, there are some unscrupulous companies/individuals who would have taken advantage and said it needed changing. In fact some others would also wildly put it was all down to the mains issue thinking the builders insurance would foot the bill.
I've read thousands of engineers reports for billing purposes and that reads to me as thought it was wrote after some specific discussion around the cause, in fact I'd wager it was written after the engineer felt pissed over something.
Again, just ime of course.