I don’t know that the WP_MEMORY_LIMIT supersedes PHP upload limits. If not, it would be necessary to set the limits in the php.ini (or supersede php.ini with .htaccess) as well. In the case of an update to php.ini, it may be necessary to restart the web server after the change. [BTW, there is only one php.ini that the web server reads on startup. You must modify the correct one, not make a new one elsewhere.]
For a multi-user WordPress, WP limits may be set using the Dashboard: My Sites -> Network Admin -> Dashboard -> Settings -> Network Settings [scroll-down to the “Upload Settings” area]. AFAIK, this is still limited by PHP configuration, so both must be considered.
The default limits are quite conservative
