remove_action returns false regardless of priority

I am writing my own plugin to customize a public plugin. My plugin is doing this customization by removing an action via remove_action() and adding it back with my customized code. The code below illustrates just the removal part in a testable way. Obviously both plugins need to be active to illustrate the problem.

First, the original plugin:

My plugin to modify the plugin above:

Why is remove_action in the “CUSTOMIZE” plugin returning false? I’ve tried changing the priority but I get the same result.