FYI, there's simply a bug here. It has nothing to do with the "overwhelming force" logic at all, even. In Spikey's game, for instance, there are simply idle threat units sitting ALL OVER the map doing nothing. Literally, there's a few hundred ships on each of his planets, and just a few thousand on the key ones.
Whether or not the "overwhelming force" logic needs any tweaking is impossible to tell very well at the moment because it's hugely obscured by this (though I've been working on that logic all day, anyhow, as it did need to handle some specific cases a lot better).