Cautiously optimistic means making sure you're actually seeing if folks are Nazis before calling them Nazis. They're saying let's sit at the table and leave the table if we see Nazis.
You again have provided zero arguments as to why a wait and see approach causes any problem. If the defederate a week after Threads launches ActivityPub, what horrors would have occurred that can't be undone? And if it's as bad as you suggest, it'd likely be only hours after launch.