The scare part refers to warning dialogs that iOS would pop up if you used a link to an external web browser to collect payments. It would warn you that Apple wasn't running the payments and use scary language warning you about potential fraud, etc. to try to scare people away. They would also demand a 27% fee from developers for collecting money on the web outside apps if users were paying from following that link.
As a developer, you are allowed to put only one 'link' (not a more user friendly button/supporting text) to your own website for any purchase, it has to be a small font, and not in the checkout flow that takes to Apple pay. Which in itself is very scummy to begin with.
To top it, when the user actually sees that hidden 'link' and clicks on that, the user is put up with a big screen of message that the external website doesn't have security, privacy etc.
How does Apple scare customers? By giving them a warning about sideloading?
A warning hardly seems monopolistic? Unless I'm misunderstanding your post