It's not clear that the Guardian owns the rights to relicense the assets, especially since they apparently were included by accident. If they don't own the licensing rights then the apparent license would be invalid. So for all practical purposes for third parties that might be interested in using the assets, I think they would have to be considered them improperly licensed regarding this Apache License "release" until The Guardian shows that this is not the case. And given that it was an accident, I don't see why they would do that.

[edit: clarity]

The law is not a magic spell, where an extra eye of newt at the wrong time can magically transform somebody else's property into communal property.

The Guardian's license most likely doesn't cover those files, as they are not owned by The Guardian.

They may not have had the right to release these fonts under the Apache license in the first place.