As long as the native aliens were not idiots and posted proper scouts and listening devices in the Screed Tanner System, then they most certainly detected the human arrivals in advance.
Even if the ships of the Premier Fleet and the Bluejay Fleet maintained a high warp factor as they moved closer to the center of the star system, the fact that they were engaging in a form of superluminal travel in realspace did not mean that they could reach the native alien stronghold before the light of their passage got detected.
That only worked if the native aliens were completely negligent in occupying the star system, but that was clearly not the case.
Stealth ships such as the Morpheus had already observed the native aliens kicking up a storm shortly after the arrival of the two fleets.
