He is incorrect.
You can copy your index.php file in your theme directory to page1.php
inside of page1.php put this at the top:
<?php
/*
Template Name: Page 1
*/
?>
Then remove the loop from the template you made (assuming you dont want this page to display posts and only want it to display your plugin), and place the function for your plugin in place of it.
Then go to the admin page of your site and “Write Page”, don’t forget to choose the “Page 1” template for your page.
Many plugins can be used this way. custom archive plugins, the itunes plugin from lucky33, etc..
If you dont want your plugin to run on index.php but do want it to be on the individual post pages you can copy index.php to single.php(included in kubrick theme already) and place it in that file.
You can also call the function from inside of a post by using a plugin like PHPExec https://priyadi.net/archives/2005/03/02/wordpress-php-exec-plugin/