The problem is that the way he clarified it is not what the rules say. The exception in rules is not reiterating anything, it is changing the general rule of performing as much as you can - if the effect is playing a card (from hand, discard pile, adventure deck, other card - anywhere except draw deck), you must be able to perform it and you must perform it.
So, with examples:
You can't exert
Treebeard, KotW only to destroy your opponent's condition (but you can exert him only to play your condition).
You can't discard cards from hand using
They Are Coming when your discard pile contains no
Orc you can play.
Similarly, you can't discard cards from hand using
Gorgoroth Sapper if you have no
Orc stacked on a site you control that you can play.
And you can't exert a Hobbit using
Hobbiton Woods if your site 2 already is on the adventure path.
But you can use
The Prancing Pony* to add as many burdens as you want (because it allows you to play a card from draw deck, and this is exception to exception, mentioned in rulebook), no matter if you have Aragorn in your draw deck, on the table, somewhere else or not at all - of course if during adding the first burden you have at least one Aragorn in draw deck, no Aragorn in play and nothing else stops you from playing companions (for example you don't have 9 of them in play).