These were, quite frankly, the last words that Ryu expected to hear. No matter how stoic he usually was, his eyes couldn't help but narrow. However, there still wasn't any part of his reaction that wasn't perfectly curated.
Being entirely expressionless in a situation like this one would be just as bad as having a fierce reaction.
The Chaotic Silk Meridians held great significance to the Fiends. It was the bane of their existence, and in other ways, the greatest treasure of their race. It was impossible for anyone to react indifferently to such words, especially if they were Fiends.
If Ryu were to react indifferently, then it would appear to be forced, and that would only make him even more suspicious than not.