They said something about the infringement on the runway splay by a fence.
But of course when the taxiway is operational, aircraft taxiing will infringe on the splay of any aircraft landing , so thats a a bit of a furphy.
When airport people talk about runway splays, they are generally taking about the approach surfaces off the ends of the runways. They are the ones mostly affected by obstacles. The side transition surfaces are the ones affected by taxiing aircraft, and transitory obstacles like those are generally ignored (although addressed by CASA in the licensing). It's permanent obstacles (like a fence or building) they worry about.
Keeping the "splay" of a displaced threshold clear while constructing a parallel taxiway to the closed runway prior to the new threshold may be too onerous, or impose such restrictions that the necessary threshold displacement won't give practical lengths for some necessary operations.
It may simply be that in this case closing the runway for six weeks is cheaper than going through all the B/S of putting in, maintaining, and monitoring a displaced threshold and nearby worksite on an operational airfield.