I'm suspicious about this, too.

It was a slightly different situation, but a couple of years ago I made a booking where there did seem to be a married sector issue: LHR-FRA-HKG was offered online but not LCY-FRA-HKG, even though there was plenty of availability on LCY-FRA. Moreover, it wasn't possible to book LCY-FRA-HKG by phone because the agent could not see FRA-HKG when searching sector by sector. (I think that married sector issues have raised their heads for others with some CX bookings.)

However, there was no problem booking LHR-FRA-HKG online, and then phoning to change to LCY-FRA-HKG, even though I could not have booked the latter to begin with.