But all VDSL2 modems are required to support downstream retransmission (G.INP) to comply with the mandatory requirements specified in BT SIN 498. And it's only downstream retransmission that was being enabled. There shouldn't be any devices which don't support downstream retransmission, so arguably they shouldn't have to support non-compliant devices.
Upstream retransmission is optional in SIN 498, which is why it is necessary for the DLM to be provided with data on if upstream retransmission is supported, and then only enable it if it is supported.