8 Year timeskip

That doesn't work as an explanation because Sten was suggesting the intelligence strategy as a junior officer around 9:11, well before his meeting with the Hero in DAO. However, it can be explained by the fact that dealing with the Arch-demon required a different strategy to the one he was recommending to his superiors. The delay in attacking the Arch-demon was because the Hero didn't have a big enough army and so needed to recruit allies. This is rather different to suggesting sending agents out undercover to discover as much as possible about your enemy before embarking on a major campaign against them.  The Grey Wardens already knew all the needed to know about the Arch-demon and the darkspawn.

Up until DAI, the Qun were not really big on forming alliances. So far as they were concerned everyone else was Bas. The Llomerryn Accord was just a tactic to give them time to regroup and focus on the major enemy, Tevinter. Sten says it was regarded as simply a piece of paper that seemed to keep the other side happy but he hinted that they didn't consider themselves bound by it and would break it when it suited them to. I just questioned why the Antaam would do this after embarking on a major assault against Tevinter, when it would split their forces and bring the other nations in on the side of their enemy. The alliance with the Inquisition was simply one of convenience too. The Qun were worried about the flow of red lyrium to Tevinter and knew that the Inquisition was likewise concerned about the substance, so it was an alliance of convenience which the Qun would use so long as it was in their interests to do so. However, they continued to work against the southern nations and the Inquisition behind the scenes through the use of their agents in the field, just as Sten had suggested should be the case.

From what I understand about the history of Sten, up until he made his suggestion, the Qun had been mostly about periodic assaults on mainland Tevinter that had been for the most part successfully repelled by Tevinter's superior magical power. This wasn't sufficient for Tevinter to go on the offensive against Par Vollen, merely to keep a sort of stale mate situation, with the ebb and flow mostly occurring on the strategic location of Seheron. What Sten proposed was using agents to discover as much as they could about the enemy before undertaking the next all out assault and also to undermine the enemy from within with a network of agents across Thedas working undercover. That is where people like the Iron Bull and Tallis came in. This strategy almost paid off in Kirkwall and would have succeeded but for Hawke. It also nearly succeeded with the Dragon's Breath plot but was thwarted by Solas, owing to his loathing of the Qun. However, it did work when they directed their attention back to the Tevinter mainland as it would seem it caught Tevinter on the hop. Hence the initial success in sweeping across eastern Tevinter. However, I imagine that if the Antaam have broken with Par Vollen, eventually that progress would slow because they would no longer be getting the co-operation of the Ben'Hassrath, who were responsible for the covert network, plus Tevinter finally getting their act together and mounting an effective defence.