My understanding is that if the tick was only a vector between two humans (for instance, malaria isn't a zoonosis, because the mosquito is just transmitting the disease between humans), then it would not be a zoonosis. But in most cases with Lyme disease, the tick is actually transmitting the disease from another animal to a human, therefore it is indeed a zoonosis.[1]

[1]: https://en.wikipedia.org/wiki/Lyme_disease#Transmission