Is the rapid ice bath chill post-sous vide actually necessary when putting the meat/fish in the freezer?
Yes, fast chilling is critical because it minimizes the production of new bacteria and potentially deadly toxins from the spores of anaerobic pathogens such as C. botulinum and C. perfringens. The spores are not killed by the low temperatures usual in sous vide, and the oxygen-free environment of the bag brings them back to activity. But they require time and temperature to become active and dangerous, so fast chilling -- either refrigeration or freezing -- and limiting the refrigeration time (to about 5 days at around 5°C/40°F, longer for lower temperatures) are absolutely critical to deter them.

The question starts with a situation that is different: cooking in an oven, which is not an anaerobic environment and thus does not trigger the same activation of the spores' lifecycles as cooking in a vacuum does.