I think I remember Dirk mentioning that it is very difficult to actually cull your deck down to less than what you started with (though possible with some quirky card play). I certainly have not been able to. There might be a way but for all intents and purposes just ignore the last part of that sentence. I think it was a way to cover their bases should that situation ever happen.
Steven is correct here. Similar to Magic: The Gathering, we know that more luminaries are coming so we wanted to write rules that would hold up even in those future cases. Of course, now that we are re-writing the rules that small bit of foresight won't matter. :-)