Those files were updated in going from 3.209.16 => 3.209.18. You can see the change log at https://github.com/aws/aws-sdk-php/blob/master/CHANGELOG.md. Normally I would say that changes wouldn't affect XF, as XF wouldn't use more than 1% of the SDK, but these releases have "Improved error handling for failed writes and appends on unclosed streams" on S3, so if anything, will make it ever so slightly more robust.
As for your initial errors. Maybe a caching issue on your end? You use opcache? Its possible that a common library was cached that defined the boolean_value() function, however as the old sdk was more than 3 years old, that function may not have existed previously (fips didnt). Did you restart php (e.g. php-fpm) after the first update?
I do you memcache so that might be an issue, I hadn't thought of that. I hadn't updated the AWS SDK on the system for a while and not too familiar with it. Thanks for the details and suggestion! I'll try another re-install during a slow traffic time and see if reloading memcache is needed for the upgrade.