Data Transfer OUT Charges Through TGW in Another Account

Who is responsible for data transfer OUT charges when the transfer is originated by a VPC in one account, and that data passes through a Transit Gateway in another account on its way out through a Direct Connect (DX and TGW owned by the same account)?

Details

With the recent announcement on Direct Connect granular cost allocation (https://aws.amazon.com/about-aws/whats-new/2019/10/aws-direct-connect-aws-direct-connect-announces-the-support-for-granular-cost-allocation-and-removal-of-payer-id-restriction-for-direct-connect-gateway-association/), it is stated that we now "allocate Data Transfer Out charges to the AWS account responsible for the Data Transfer."

I have a customer who acts as a transport service provider, and they own multiple DX connections that they plan to connect to Transit Gateways via a Transit VIF and DX-GW. The customers of my customer have VPCs in separate accounts that will be connected to my customer's TGW. With my customer owning the TGW, I am unclear on whether the cost allocation per the above announcement will consider the owner of the TGW responsible for the data transfer OUT, or whether that responsibility will be attributed to the owner of the VPC that originated the transfer.

Also, if traffic routes through a firewall or IPS in Transit VPC owned by the DX an TGW owner on the way out of AWS, will that change the consideration of cost allocation?