But Sloan's only real interest with DS9 was with Dr Bashir, why would he be tailing Sisko and Odo two years before he began pursuing Bashir?
How does one keep a cabal larger than three secret for centuries? It's not possible.
Really? You want to go there? FTL travel isn't possible, instant translation of a newly discovered alien language isn't possible, Star Trek is filled with things that aren't possible. That's why it's fiction.
But in the end, it comes down to intent. The intent on DS9 was that Section 31 was a 200 year-old organization, as was the intent on Enterprise. Maybe it doesn't fit your idea of "possible" but it is canon. By the definition of everyone who has the authority to say so.