If a deflated Sisko did concur, or did not protest, he would know that he, Sisko presented information that he didn't believe in himself. What good is that? Room for doubt.
I'm not pointing to the conclusion necessarily, but to Sisko's confidence, or apparent lack of it before Vreenak actually took the crystal away to inspect it. Vreenak probably weighed all the information available, not only the crystal, but the plausibility of the scene itself, and Sisko's rather ambiguous demeanor. Calling the bluff would be final confirmation if he did not protest.
If he did deem it 'real,' or was uncertain regarding it's authenticity, the next step would be independent confirmation from Romulan sources, and debate in the Senate. That is what Sisko would have wanted. He wouldn't have simply voided the treaty without investigating further.
Geez, when I think about it, Garak's way would have been the only way the thing stood up to scrutiny. Or, is that what Garak said? I'll check that. That may be a(nother) flaw in my thinking.
Garak says that he hoped the data rod withstood scrutiny, but I think we can say Garak's method was the more certain method.