Here's the thing: it's really, really not always going to be true. Because if we use Goddess Mode as a coup de grâce rather than an opening move, we can reliably expect to wipe all enemy assets from the board before passing out and our opponents will still not have a viable estimate of how long we can sustain the state. That's half of why it worked out fine when Panda flared Goddess Mode to run off Magnus; she popped it, he rabbited, and Chaos didn't have anybody on the field to notice when we went into a seizure and passed out for a couple days (the other half is that we were in motion and by the time they could follow up on that flare we'd already have been gone). The incentive structure I laid out is only true if we pop Goddess Mode and pass out before clearing the field, and that does not always have to be the case, even before we do more work on refining our control of Goddess power.
If our enemies don't know how long Goddess Mode lasts or how incapacitated we are when it ends, they have to consider the possibility that sending an attacking force to follow-up will just run into the same active buzzsaw. If they do know how long it lasts because we couldn't make it through all their assets in play before passing out (and if they have close enough eyes on to see us pass out, they have a good sense of just how incapacitated we are as well, which would be even worse for us), that consideration will no longer apply and cannot thus stay their hand.
In short, we need to be strategic about when we apply Goddess Mode, not just where. And "when" should meet BOTH of two conditions, at least until we learn more about how to control Goddess Mode:
1) We should be confident we can rely on definitely or almost-definitely being able to clear all enemy assets from the field before passing out.
AND
2) We should be planning to move elsewhere shortly after triggering it, if we aren't already in motion already.
If Condition 1 isn't met, then the dangerous incentive structure I already outlined comes into play. If Condition 2 isn't met, then the surge of attention from all of the Four will make everything else we want to do in the region harder and more dangerous - we should remember that Chaos has ways of making life hard that don't rely on confronting us personally directly, as well.
Okay, I understand now, and I think I agree. I was wrong about the incentive structure, because attacking Pandora while she's unconscious requires you to know that she's unconscious and it requires you to know where she is.
We should keep in mind that Khorne may not
care. A Khornate Greater Daemon might get super angry at the "Little Anathema" and just throw everything at her last known position, regardless of how strong she is or whether she's even still there.
The Tyranids are a different story. They have less experience with gods, and their most likely reaction to "Pandora nukes surviving Tyranid hordes" is "try to develop new strategies while evolving anti-psyker strains of Tyranids". Which is not going to be fun, but that's a problem for another day. Based on my limited knowledge, they're not as likely to
banzai charge into the position of a major defeat; they'll want to come back prepared, while Khorne's followers are not always devoted to common sense. He cares not from whence the blood flows, after all.
An ideal deployment of Goddess Mode will meet both of your conditions. However, I am open to the possibility that we might have to deploy Goddess Mode is less than ideal conditions; if it comes down to a choice between "lose Baal" and "go nuclear", I would be willing to go nuclear even if there are still enemy observers in the system afterwards. Also, if we could wipe out only ninety percent of enemy forces on Baal with Goddess Mode, and doing so would save us from having to fight a grinding, bloody campaign with millions and millions of casualties, I would be willing to go nuclear under those circumstances as well.
Chaos knows that we can enter Goddess Mode. They also know that we can't stay in Goddess Mode all the time, or we would just proceed from battlefield to battlefield like a terrible avatar of divine wrath. Though I entirely agree that I want to conceal the specifics of how long we can sustain Goddess Mode and how incapacitated we are afterwards, I don't prioritize doing so over every other consideration. "Wipe everything and then rabbit" is the ideal strategy, but it isn't always possible.