The root cause appears to be that the ca-certificates RPM did not install because the dnf_aixtoolbox.sh script is not able to download the dnf_bundle_aix_73.tar bundle because ... the ca-certificates RPM is not installed. Got a chicken-and-egg situation now. The line
export PERL_LWP_SSL_VERIFY_HOSTNAME=0
used to allow lwp-download to download dnf_bundle_aix_73.tar in dnf_aixtoolbox.sh, but it's not working on AIX 7.3 TL3.