Views Data Export does not render your results in a traditional way, so render hooks are not executed. But you should be able to alter results using hook_views_post_execute:

This hook is called right after the execute process. The query has been executed, but the pre_render() phase has not yet happened for handlers.

 

Adding output to the view can be accomplished by placing text on $view->attachment_before and $view->attachment_after. Altering the content can be achieved by editing the items of $view->result.

Emphasis mine - that's what you should to to alter content not only for display purposes, but in generic, usage-independent way.

Views Data Export does not render your results in a traditional way, so render hooks are not executed. But you should be able to alter results using hook_views_post_execute:

This hook is called right after the execute process. The query has been executed, but the pre_render() phase has not yet happened for handlers.

Adding output to the view can be accomplished by placing text on $view->attachment_before and $view->attachment_after. Altering the content can be achieved by editing the items of $view->result.

Emphasis mine - that's what you should to to alter content not only for display purposes, but in generic, usage-independent way.