What does is_page_template() compare against?

Your condition should be written like this:

if (is_page_template('path/file.php')) { 
    // Do stuff
}

I believe the confusion is a result of two things:

1. The docs refer to “name” ambiguously. Specifying “file name” would make the documentation much more clear.
2. The code behind is_page_template() shows the get_page_template_slug() function at its core. This function actually returns a file name, not the template slug. https://codex.wordpress.org/Function_Reference/get_page_template_slug

When specifying an argument for the is_page_template() function (as in the example above), the file path is relative to the theme root.

This function will not work inside the loop.

EDIT: an important issue to note here as well. The is_page_template() function will return empty/false if the page is using the default template from the hierarchy. If a custom template is not assigned, you must use another method, such as basename(get_page_template()). See Jacob’s answer here for more details: https://wordpress.stackexchange.com/a/328427/45202