@Terithian well, it's cause Eldan knowing the decree is fake relies on you as the reader remembering details from Chapter 9 and Chapter 17.
Eldan heard of Diaz's deeds through stories and plays in Chapter 9, as a massive hero for the kingdom, who even the king respected. Receiving a decree that amounts to a death sentence to the kingdom's hero, declaring him a rebel, is such an obvious fake that wouldn't have fooled even the commoners.
Chapter 17 expands on how much the king trusted Diaz, to where even the first prince was put under Diaz for a few months, and the whole point of heading to the frontier was to guard it, not being banished.
Now the thing with the king's seal, is that the contents of a decree can be obviously false, but the seal turns it into a legal document. Imagine if a document declaring a baby to be a dangerous criminal and was to be hunted down was passed into law. And as others have said, it'd taken time to verify the decree and seal with the king, meanwhile this idiot princess has an army at his fiefdom, and being the unstable psycho she is, might very well use the decree to declare Eldan a rebel in league with Diaz and cause massive dmg to him. Especially since Eldan just fought a civil war against his father and brother, and likely was in a bad position to fully resist a potentially hostile army that was already inside his city.