Old posts and comments will be seen, new ones won’t be seen. One-sided defederation is like two linked servers delinking from one another - you keep the old info but you basically desync and one side can’t see the other if they add new stuff.
That’s why I have accounts on both kbin and l.world - so that I am able to access more information as it gets updated. It seems the influx of new users is leading to a fracturing and a defederation of the biggest instances, so having one account for all of the threadiverse is not viable.