Hair is tissue and doesn't contain DNA. But that's probably not what they are talking about. DNA can also degrade. This is all speculation until it gets into court and the lawyers go at each other's throats.
I assumed that the reference to 'tissue' was the common usage definition of tissue. Commonly hair is simply called hair.
What time span was there between the event and the evidence collection? What's the likelihood for contamination from others in that locale at the time of the event?
Too many questions to resolve here I'm afraid.