Because it is a flag set per exchange and the BT Wholesale checker does not hide it on a per cabinet basis
I believe it is per cabinet. I am at EMBOSTO, and my cabinet, PCP64 only shows Copper Multicast, but the cabinet just down the road (PCP28) which does have a fibre twin, shows Fibre Multicast on the BT checker. I always thought if it shows Fibre Multicast but no Fibre speeds because the distance was too far to warrant good enough speeds.