Is the bilateral restricted on slots held or flights operated?
The airline holds the rights, and there is no enforcement to get them used (unless an airline appeals at their non usage). The best example is to research the 1998 decision to award Virgin Atlantic the then 2 weekly rights for PVG flights, that shows how it all works (openly available to the public).
Therefore, yes, it is entirely possible that nothing has stopped Air China opening MAN in the past (maybe holding out for the LHR slots, after all LGW was purely a waiting room), but, on the flip side, if all the rights were held by other carriers, then yes, Air China were effectively blocked.
However, it still does not alter the fact that only 1, 3 at a maximum if the CA cargo rights get transferred, slots are left to operate UK-China flights at this time, ergo, bilaterals are currently an issue at this moment in time, when they are being touted as a non-issue.