True, if this wasn't a direct reaction to something that wasn't known to the general public. What if that attack was a reaction to Imperial Fleet actions on the frontier? A botched assassination attempt? Other economic warfare? Any one of a number of possibilities exist, if you are open to the idea.
At that point, you're on a "fishing expedition" for a conspiracy theory ... and the only fish you're going to find are going to be "off the reservation" (so to speak).
The basic flaw in that line of argument(s) is ... where is the EVIDENCE for any of those possibilities?
In case you missed it, my core assumption was that what we "know" as canon might not be the whole story, or even true.
Even allowing for the
Unreliable Narrator phenomenon, the notion that the Imperial Emperor (Strephon at the time of the Fifth Frontier War) "needed" an external enemy (in this case, in the Spinward Marches) to deflect attention away from a rebellion somewhere else simply doesn't pass the laugh test.
Let's say ... just for the sake of argument and illustration purposes ... that "things are not going well along the Solomani Rim" of the Third Imperium (the
Solomani Rim War was 990-1002, so "it's about time for another flare up"). So in order to distract the citizens/wag the dog ... Stephon "arranges" for there to be a Fifth Frontier War in the Spinward Marches.
That could work on a planetary scale, with near instant communication networks.
It flat out doesn't work once you factor in the Tyranny of Distance between the Spinward Marches sector and Core sector.
I mean, there's a reason why the
Fourth Frontier War stared and ended before any messages could be sent TO the Emperor and any decisions/orders from the Emperor could make it back to the front lines before an armistice was reached. So there's already been a "test case" proof of how actively the Emperor can intervene in the region of the Spinward Marches in response to unfolding events, and the answer is "NOT FAST" (enough) to shape events.
What if the Emperor is trying to deflect internal rebellion and give the Imperium a convenient enemy?
There IS an opportunity for the Emperor's authority to be MISUSED (and abused!) by an unscrupulous individual, in the context of the Fifth Frontier War. But in such a circumstance, the blame/responsibility should not be laid at the Emperor's feet.
I'm talking about an
Imperial Warrant ... which first appeared in LBB A1
The Kinunir ... and which actually became an important point in the FFW game published by GDW.
If such an Imperial Warrant were to fall into unintended hands ... there could be extremely far reaching consequences. That individual could "speak for the Emperor" (and thus wield the Emperor's powers and authority) without being ACTUALLY loyal to the Emperor (or the Third Imperium). Such an Imperial Warrant would act as a "trump card" which (in the wrong hands) could be misused and abused.
However, that kind of scenario is more of a "loose nukes" type of situation, rather than some kind of perfectly orchestrated puppet master scheme with the Emperor pulling all the strings (directly). At best, the Emperor would have to be playing a
Xanatos Gambit for anything like that to be plausible (and even then,
Xanatos Roulette is the more likely trope) ... which is just "stupidly unlikely" given what we know about Emperor Strephon (which is to say, NOT MUCH, but he didn't see his own assassination coming, soooo ...

).