Exactly. The minute you claim you are stubborn, your opponent will ask well how is that? At that point, you would have to reveal the assassin with the crown of command to gain the benefit of the item. At least that is how I see it and would play it.I think what he's saying is to have an Assassin, tell your opponent that you have an Assassin, but then never place the Assassin in the regiment. Since your opponent can't target the Assassin (he's not there) you would never lose Stubborn.
I don't think it works though. The Assassin is not there - he can't be targeted, he can't be affected by Spells, etc. Likewise, he can't lend his fighting ability to any of the models, can't use his items, etc. Basically, he's just not on the table, he hasn't been deployed yet.