Why do I need `zend_extension = xdebug`?

You don't need to add zendextension = xdebug, if your current full path line is already working.

1. extension = ... -> normal PHP extensions
2. zendextension = ... -> Zend Engine extensions (low level hooks into the PHP engine)

Xdebug is a Zend extension, so it must be loaded using zendextension, not extension.

zendextension=opcache
zendextension="D:\...\phpxdebug.dll"

Why does the wizard suggest it

The Xdebug Installation Wizard gives generic advice, assuming you might install via package manager or PHP already knows where Xdebug is located

So it suggests zend_extension = xdebug

That only works if The DLL/SO is in PHP's default extension directory and PHP can resolve it automatically