So 12M made a point of mentioning that the castle had a strong threshold on account of how many neighbor families Harry had welcomed in while their homes were uninhabitable. Including the roof, per the gargoyles.
Previously, we've got WoJ about vampires and thresholds that the White Court could cross a threshold uninvited but they'd be barely able to use any supernatural mojo, the Red would be nearly disabled by doing so, and the Black just couldn't.
So ... what the hell?