For the record, Chinese scientist thinks 700 million is the ideal population size that China's natural resources can sustain.
This means decline is the intent. Whether it can stabilize the TFR to maintain 1 billion or 700 million is the issue (i.e., irreversible or reversible instaneous TFR rate) not the fact that the population is decline, which is the actual intent of the policy.
With Technological and military growth, China should not have any problem securing resources from all over the world to sustain its population. China has this fear of getting blockaded by the US due to the dominance of the US navy around the world. But the world is getting more and more globalized and resources are no longer produced in the same country where its consumed. So, resources should not be the limiting factor for China's population sustainability.
If China grows in terms of GDP and military power as we expect and maintains current trends, it should not have any problem mitigating any kind of US naval blockade. In fact, if China can realize its true economic and technology potential, China's total GDP should be 4 times as much as US due to 4 times bigger population. With this large GDP, it can easily maintain 4 times more powerful navy and probably blockade the US instead.
All of this is only possible if China maintains its current population advantage. If US closes that gap due to immigration and Chinese population decline, then US will most likely be able to maintain its dominant position in the world and China will be under more and more pressure from US growing GDP and military power.
So, if China has made reducing population to 700 million a target, then its a big blunder. I doubt they will maintain this kind of policy. I expect the more China's GDP grows, the more they will have confidence in their military and economic influence in the world. Then they will not have this kind of anxiety to keep 100% self-sufficiency in food and other natural resources. Then we might see policy change that tries to raise birth rate to maintain population size.
The new cold war between the west and China will be fought on all domains and even the slightest bit of advantage will be utilized. US think tanks and other prominent neo-conservative thinkers are already calling for more immigration to raise US population size so that they can blunt China's population advantage. China will not be able to tolerate losing this advantage as well. They will have to change their direction.