And as true as that is, the Gravemind's intelligence doesn't, at all, appear inadequate to do so.
Regardless, if the Flood are intergalactic somewhere in the Halo universe, it's frankly shocking to me that there would be an invasion force so easily wiped out. Any nearby supernova event or a more measured, conservative approach towards analyzing the Flood's homeworld in this galaxy would have resulted in no infestation. If it were as easy as not landing anything FTL capable on the planet, then there would be no Halo Array, no Conservation Measure.
It simply boggles my mind that this can be considered an invasion. They were trapped on that world from what information we have.
Why does it have to be an invasion? The Flood doesn't require tactics. Once they acquire intelligence they might use it, but until then they are a pandemic. Wherever it touches turns to Flood, case closed. So they didn't have an invasion fleet. They didn't need an invasion fleet. All they needed was for one spore, just one, to run into a lifeform anywhere in our galaxy. Once it has that foothold, the Flood will acquire more forms until it gets the compound mind going.
Why do they need FTL? What stops them from shotgunning out spores towards the galaxy? So what if it takes hundreds or thousands of years? They don't care. Throw spore infected meteors at the galaxy. One of them will hit something sooner or later.
If you look at it, it makes a lot more sense for the Flood not to have been on the planet before. Think about it, in our time, the Flood was locked down in the best security the Forerunners could come up with. The second they got out, it was madness. In their time, if they were on a planet previously, in anything LESS than forerunner security, they would never have been dormant in the first place, they would have infected something as soon as life was on the planet. Obviously there wasn't any forerunner security, so they must not have been on the planet or it would have been overrun. The Forerunner scouts would have run into a planet full of Flood, not an anomaly.