Why Isn't The 'map Colors' Fixup Removing Background Color Behind Text?

I'm trying to remove the purple color that backgrounds quotations in this PDF. Unquestionably, it's too inefficient to delete every instance of the purple background singly and manually!

1. I don't understand Dietrich von Seggern's abstruse answer. Please explain like I'm 5?

2. Please see the GIF below. Here's the PDF. I can't remove the background purple with CMYK = (16, 16, 4, 0).

https://s6.gifyu.com/images/BEK---Cant-remove-purple.gif

3. This other PDF has this same glitch. I can't remove the background grey with CMYK = (0, 0, 0, 10).

 

Moderator — Apology! Can you please correct my initial post as follows, then delete this thread?

1. The URL to the screenshot is
https://s6.gifyu.com/images/BEK---Cant-remove-purple.gif.

2. The correct URL for this PDF — call it BEK — is https://mega.nz/file/yho0RQCL#PWD6T9TUtyrl0kDwpkqPp0XkcXiSeHwckqc9ZZRyqcY.

3. For McColgan — https://mega.nz/file/KlgETAbC#xA6Ll1h4vUDFcdL6aqsJK8S_dJrD81Zf89TE-9a5m-U.