Yes, they have a history of hitting each other. I don't know if there was DV involved, I think a lot of people are projecting their personal history into what they see on the bodycam video, but it does appear that there was mutual battery (when domestic partners both use any deliberate and unlawful force or violence against each other). So, with the documented history of mutual battery, even if she had his DNA under her fingernails, can it be proven that it wasn't from a fight they had before he hiked off? I can see reasonable doubt with that too. Although, any DNA under her nails was probably outside too long in the elements to be useful, from what I've heard experts say.
ETA: It's interesting that the couple who picked BL up and the other woman who picked him up hitchhiking didn't say they saw scratches (defensive wounds) on his face.