Someone said "everything after the master socket is at Openreach cost?"
To which you replied Yes.
MHC may be being pedantic but is sort of right, because 'after the master socket' depends on your view point, there is after the master socket with respect to extension wiring, and there is after the master as the wire heads off to the exchange.
There is also a scenario where costs would fall onto end-user for repairing a fault even if outside the property and heading towards the exchange, e.g. overhead cable damaged by rubbing against a tree branch on a users property, i.e. you are expected to keep a tree trimmed back as it grows, or another gardening example, strimmer damaging cable attached to outside wall of property, or over enthusiastic digging that cuts wire, or damages duct and cable.