The rules state
FIRST discard Treasure Card(s) with a value of at least 1 point.
THEN take a Sneak Test.
The wording on this is vague. I'm assuming that you discard a single treasure card with a gold coin value here and not all of your treasure cards with gold coin values.
The card would be clearer if it said, "FIRST discard a single Treasure Card with a gold coin value."
The "(s)" is confusing because it implies you might need to discard more than one.
After reading a bunch of summaries here and thinking it through, discarding a single treasure card makes sense. I played where you had to discard them all and it made more sense to let the Dragon attack you in some cases.