And that's the part where I disagree. If $a->{b} doesn't exist, we know that $a->{b}{c} cannot exist,

But he isn't checking if $a->{b} exists, so it's possible that $a->{b}{c} can exist. Perl is being asked if $a->{b}{c} exists, and it's being done a few steps after autovivifying $a->{b}.