I'd disagree with this assessment. The slots don't belong to Australia per se. Japan has agreed to allow Australian carriers access to three slot pairs at Haneda on the basis of reciprocity with Japanese carriers. However, Australian carriers must still apply for and maintain the specific slots through the normal slot allocation process. All IASC does is decide who gets the right to utilise those slots. Non-utilisation under the regular Haneda slot rules will result in the specific slots being lost back to the pool and the airline would have to reapply for a new slot pair, possibly at a different time.
More concerningly is that the right to those slots aren't guaranteed. Qantas highlighted this in their initial application for the rights to IASC back in 2019: "Any potential delay to the commencement of services by a carrier applying for capacity or disruption to operations will place these highly valuable slots, future access and the strong, longstanding relationship that Australia maintains with Japan at risk. This could set an international precedent which negatively impacts Australia’s prospects or leverage in negotiations on future access with Japan or more broadly with other strategic partners."
It led to the IASC asking the department for legal advice on the matter. The department's advice on the matter was that an unutilised slot "would ultimately be a decision for the Japanese authorities". While no other Australian carrier could substantially challenge them for the right to the slot, Qantas can certainly lose access to their allocated slots and Australia could lose the right to their 3rd slot. Furthermore, non-utilisation would de facto undermine further access to Haneda slots.