I never ran across this until just today when I was trying to help someone else:
The holiday integration allows you to select your country and subdivision within that country. That’s it. Want to add or exclude holidays? Sorry, nope. Want to customize whether “optional” or “government” holidays are included? Sorry, nope.
The workday integration allows you to do all those things that the holiday integration doesn’t.
They both utilize the same python holidays module, so it should be possible for some smart person to give both integrations the same capabilities.
WTH!