The latest DWM
quotes Moffat that they have "just finished shooting the actual completed ending
There's no telling when Moffat would have written that, truly, but I suspect it probably wasn't before Christmas when series 7 wrapped.
This leads me to wonder if he's alluding to filming for the finale that was done early in the shooting block for the anniversary special. Or possibly filming that was done on An Adventure in Space and Time
's sets. Neither would surprise me.