It may be that they didn't like the results of either pair and ran a thinner new cable with less pairs on, then connected 1 pair of that to the BT66 and PCP.
It doesn't work like that. If they don't like the results of a pair test they will swap the pair.
If you only have 2 pairs at your BT66 and both are knackered then they would install a new drop wire from BT66 to pole or underground chamber
They then connect that to a spare pair at the pole.
It isn't a single length of cable from the home to the PCP. Each multi pair cable only goes as far as the next junction box where they are joined to the next section, and so it goes on.
It's done in such a way that they don't need to replace the whole lot but can work around a damaged section of cable.
You probably have over a dozen sections of
cables between you and the PCP.
They can test your pair at each junction box and tell where the pair goes bad.
If there were no spare pairs and yours is faulty then that needs logged and a work order raised for another team to add more.
It wouldn't be the engineers who attended your fault who did this and it wouldn't be the same day.
They will only add more pairs for any sections that need it.
TL;DR: there's no chance they replaced your entire line from home to PCP.