Xfinity Mobile is just an MVNO (Mobile Virtual Network Operator) of Verizon. So it is Verizon's towers/networks that would be providing your connectivity, not Comcast. Comcast basically just handles your billing and customer service.
I’d be more worried if they take Verizon traffic and route it across their own network, which most MVNOs don’t — but Comcast might for data collection purposes. Hopefully not!