There's not really magic with s3, you still need to name things with coherrent prefixes to spread around the load.
DynamoDB is almost simple enough to learn in a day. And if you're doing nothing with it, you're only really paying for storage. Good luck with your decisions.
I'm not going to speculate on the accuracy of 90% value, but I will say that appropriately prefixed objects substantially help with performance when you have tons of small-ish files. Maybe most orgs don't have that need but in operational realms doing this with your logs make the response faster.
DynamoDB is almost simple enough to learn in a day. And if you're doing nothing with it, you're only really paying for storage. Good luck with your decisions.