Unless I misunderstand you, no, that's not the case.  When a custom session handler is used, it is "returned" by reference as the value of the "session" element of the `initSession` hook, thus replacing the value of `self::$session` entirely.

Example:

```php
<?php

namespace OCA\SessionTest;

class Hooks {
	public function initSession($params){

		$handler = new FileSessionHandler();
		session_set_save_handler(
			array($handler, 'open'),
			array($handler, 'close'),
			array($handler, 'read'),
			array($handler, 'write'),
			array($handler, 'destroy'),
			array($handler, 'gc')
		);

		register_shutdown_function('session_write_close');

		// This overwrites the existing Session instance
		$params['session'] = new PracticalSession($params['sessionName']);
		$params['useCustomSession'] = true;
	}
}
```