Did they? Why would they have needed to? (Genuine question as I don’t fully understand the intricacies of new potentially GBR revenue-abstractive cross-border service planning phases, separate from the delivery of pre-existing cross-border services on behalf of DfT.)
It's part of the terms of the Wales and borders franchising arrangements (those that allowed Welsh government control of the service provision in the first place) for stuff that crosses into England they need DfT approval, including for applications to the ORR such as this one.
The DfT, admittedly one run under a different party, also agreed to even include Bristol in the Wales and Borders operational boundaries too, this idea of services into Bristol ran by TfW isn't a new idea that's caught DfT off guard.
Both of these are mentioned in the ORR application so aren't just my speculation, they were new information to me too.