Even when carefully following the steps to re-sign the WordPress app in this chapter, it fails with the error message
resource fork, Finder information, or similar detritus not allowed
The reason seems to be that when you view the package contents of WordPress.app in Finder, the whole bundle, including all of its frameworks, obtains the com.apple.FinderInfo attribute. But this attribute prevents signing the app.
To remove the attribute for the main app, use
sudo xattr -cr WordPress.app
However, it did not seem possible for me to remove this attribute for the frameworks in this way. 
Any ideas?
Otherwise, be warned to make a backup copy of the WordPress app at the beginning of this chapter, and do not show its package contents in Finder.