TFR is total fertility rate. GP is saying "the problem is not the overconsumption in the rich countries with low TFR, it's the poor countries with high TFR getting richer and consuming more and people migrating to countries where they consume more".

Which falls apart looking at the map of highest ghg emissions https://ourworldindata.org/grapher/per-capita-ghg-emissions vs the negative impact caused by climate change and the ability to mitigate https://gain-new.crc.nd.edu/ranking .

curtailing migration without massive reduction in the overconsumption by the rich countries amounts to having the poorer nations pay bills (via enduring disasters) for damage they didn't cause, which I find an injustice. Doing that and limiting economic growth and thus limiting the capability of dealing with the disasters would just be cruelty.

If you want to reduce migration without injustice or cruelty, you need to make sure other countries remain livable and thrive, hence the OPs hook.