The answer is the integration of land based air defence within the fires brigade, as well as the increase of major combatant numbers by eight.
Remember, for the first time in over 25 years the number of majors is being planned to increase, and doing so quite significantly. The individual capability of even the tier 2 majors will dramatically increase.
The capability of the Army's air defence is also increasing, as is the ADFs over all air defence and surface strike capability.
Add to this the pivoting of the infantry in particular, but the ADFs brigades in general, to a more amphibious structure, able to deploy with and protect the long range fires.
Don't forget the massive leaps in capability in surveillance and situational awareness.
This mission set is the reason why the ADF is now only getting one, instead of three hardened brigades.
I'm not saying it's right or wrong but this is what the DSR is indicating.