I found my problem.

The template I've been working with/updating is "classic", so I assumed that when the readme said

includes/templates/YOUR_TEMPLATE/tpl_product_info_display.php

I should put it in
includes/templates/classic/tpl_product_info_display.php

However, it only worked when I put it in
includes/templates/template_default/templates/tpl_product_info_display.php