Well... I did post a new commit earlier today saying that I'd reverted the function change... :^^;:
This is due to a PHP bug, where closures enclosed within a class method can't call protected or private methods that would normally be available in their scope. Since closures are precisely supposed to be functions that inherit their context's visibility settings, it's really what I'd call a bug.
Hopefully, this problem will only be obvious in that function. But really, enough to make you think twice about going for a 100%-closure codebase, I guess... So much for PHP 5.3 being a must-have?