I wouldn't make that inference if the provider is just restating the lab result, but even if you did, my thought is that there is no appropriate code. B96.89 is for 'other specified' bacteria, and 'mixed flora' is not specific. There is no information in this statement to indicate what type of bacteria is present or whether or not it is responsible for the infection, and I don't think there's a code for unspecified type of bacteria that would fit.
I recall this was a question that came up a lot during rollout of ICD-10 a couple years back - the ICD-10 instructions for UTI state to use an additional code to identify the infection agent but physicians often simply don't have this information when they make this diagnosis, and medical guidelines do not allow physician to test a patient for something simply because they need to come up with a code. There wasn't any coding guidance, that I know of, on what should be done in at case. The facility where I worked at the time made the determination to not code for a cause if it wasn't known and to my knowledge, this never cause a problem with denials or audit findings. Perhaps someone on the forum with more ICD-10 background can give some additional input about this situation.