How to stop Safari from hijacking Amazon app links?
It's not Safari hijacking the link but rather the Amazon app reading the link it was passed and declaring that it can't handle the link, throwing it back at Safari to handle generically.

Amazon's website declares the links that should open in the app should the app be installed on your iPhone. You can download this list yourself by going to amazon.com/apple-app-site-association.
Opening a link on iOS which matches one of these links will instead pass the link to the Amazon app.
The app ‘opens’ the link (switching Amazon to the foreground) and the Amazon app ‘reads’ the contents of the link, attempting to navigate to a reasonable page within the Amazon app to reflect the intention of the link. This uses application(_:continue:restorationHandler:).
If for whatever reason the Amazon app was unable to complete the intention of the link, the Amazon app can return false ‘to let iOS know that [the] app did not handle the activity’. This occurs within the Amazon app code.
Since the Amazon app refused to open the link, Safari is opened as a backup and navigates to the link.