Not only that, it may push Anys to doing something even more rash before we are prepared to face her. Especially if we start slaughtering the foes gathered at Luseng with worrying speed...
Heretical is slow moving and very much an indirect vector of attack.
Buffing up Zao and stopping Anys Syn's ability to erode his influence is very much blatant and far more likely to result in her attempting to escalate against him, especially if she hears we might be win at Luseng and gain our own power base, before she has even started dismantling Zao's.
Anys Syn's plan is to have us hopefully killed and at the very least occupied in the Satrapy positioning, while she focuses her attention on weakening Zao.
That's why the update was called Divide; Conquer.
If you're afraid of Anys Syn doing something rash before we're ready, visibly frustrating all her plans to stop us is very much the opposite of what we want to do.
The basic thought behind the Scarlet Flag approach to our overall conflict with Anys Syn is that we essentially just fend off this attempt to harm us.
The ideal result is that Zao loses none of his power, we come out of it with a very rich Satrapy behind our back and an established repuatation as a great Hero of the Realm.
The obvious risk here is that as it becomes more and more clear that her plan is not working at all, Anys Syn will go for another strike. This may not turn out too bad for us, given that she might make a mistake we can exploit, but, as you said, her acting too quickly is easily going to be dangerous and since even the ideal result is just Zao's forces, Uly and a very rich Satrapy against someone currently controlling the entire Realm, we'll still have to come up with and enact schemes that actually tilt the odds in our favour.
The basic thought behind the Heretical Prophecy on the other hand is that we kick off a very indirect, but potentially very threatening assault of our own.
Zao may lose influence while we do so, but the navy has always been a diminishing force
and Zao is totally going to die.
Seriously though, the ultimate result of this would be that our faction's focus moves very much away from Zao and his political influence, which does seem somewhat appropriate once we start surpassing him in power, especially given how often we've decided that his preferred course of action wasn't a good one.
And while Anys Syn will still (hopefully) get reports of our success, the fact that she is successfully acting against Zao should reassure her. The effects of the Heretical Prophecy are slow moving and subtle enough that they are anything but immediately apparent and, once they do become apparent and even recognizable as a Solar's work, they'll have enough momentum that she needs to deal with them somehow and that she can't get rid of them by moving against us alone.
The hope here is that, once things in Luseng are more or less dealt with, Anys Syn will have too much to worry about our grassroots movement to move nearly as strongly in response to our survival of the suicide posting and that, while Zao's influence was diminished, us getting much more free time again will allow us to both counteract that and increase the pressure on Anys Syn to even greater levels.
The risk that we won't actually have the time to cultivate the Heretical Prophecy is a real one, yes, but even if we don't get another chance to continue on this for quite some time, kicking off the whole thing and allowing the first step to propagate should keep this course of action available for a while and we do kind of need to find ways to strike at Any Syn, if we want to prevail against her, and the potential ultimate payoff of Heretical Prophecy is so very ridiculously good that it seems very much worth it.