The point is that take your brain, put it in an isolation / sensory deprivation chamber and your "mind" functions are entirely limited to the physicality of your brain. Once those constraints are lifted all bets are off. He introduces reasoning and decision making as functions of the brain (in addition to memory) and how the scope of inputs you can utilize is changing. We already know how people reason can be hugely impacted by where they live which limits the scope of what knowledge they have a priori. Ditto for who they know. And now we should also consider what information systems they have access to and choose to utilize.

So reasoning and decision making being functions of the brain he posits that how we do that and the results are now hugely impacted by a sphere of influence (physicality) far beyond the confines of their cranium. That influence is now decreasing our independence of reasoning and ability to form new contrarian ideas so reducing our ability to problem solve through diversity and exploration of alternatives, which even if surfaced occasionally are pounded back below grade by the hive "mind" of external influences.