What's the difference between testimonials archive and a testimonials listing page

The archive page URL is controlled via Testimonials > Settings > Post Type. It's a built-in WordPress archive type page like http://axelerant.com/testimonials-archive/. This is controlled by your theme.

A testimonials listing page like http://axelerant.com/about-axelerant/testimonials/ is created by your creating a new page and putting the shortcode and options you want on it.

Further, don't let the archive URL in Testimonials > Settings > Post Type be the same as your testimonials listing page URL. Just delete what's in the settings to reset it if needed.