The Kunena plugin does not generate a simple profile URL when linking to the profile. This can be seen here: http://test.peakbeta.com/forum/general/75-attachment-post

The profile link on that page goes to the non simple URL method. Ideally the plugin should check the setting to see if Simple URL is enabled. Or we could simply use:

Foundry::user()->getName();

Works like a charm on my site. Perhaps the Simple URL method is being checked behind the scenes when using this code?