Sisko working with Garak murdering Romulan Senator Vreenak was absolutely Section 31 material.
It’s also arguably something that Article 14, Section 31 is actually meant for. The Dominion was in the process of conquering the entire quadrant and dismantling the Federation government, among others, and they had the means and manpower to do so. That’s an extreme and existential threat. The thing Sloan tricked Bashir into doing with the Romulans, on the other hand, is just a regular Tuesday with the RSE as your neighbors and therefor not an extreme threat.
It’s also arguably something that Article 14, Section 31 is actually meant for. The Dominion was in the process of conquering the entire quadrant and dismantling the Federation government, among others, and they had the means and manpower to do so. That’s an extreme and existential threat. The thing Sloan tricked Bashir into doing with the Romulans, on the other hand, is just a regular Tuesday with the RSE as your neighbors and therefor not an extreme threat.