This was hardly a full scale attack, more like a minor incursion. In the event of a full scale attack they wouldn't have just knocked out a few planets. They would just keep attacking...
That report was summarizing the entire Borg/8472 war up to that point. Note
'Over the course of Five Months'. That is the scale on which 8472 was able to plan, mobilize for, and execute operations against the Borg, who they'd comitted themselves to exterminating.
Seven of Nine used the Borg method which she did not reveal.
It was still done using only the machinery availible to a Federation starship, specificly a technology common to neigh-all trek races. (The navigational Deflector) The Empire need only capture and study a number of Trek ships, or salvage the remains of destroyed ships, in order to learn how to access Fluidic space.
Until then, automated Defence platforms could be used to guard worlds from surprise attack. Do you really think it would take that long for a small, dedicated satelite to detect a sudden surge of energy, turn itself to face the intruder, then open fire upon it? Even if the first solvo doesn't hit, it will force the Bioships to take evasive action, disrupting the formation they need to fly in to perform their planetbusting attack. Continued firing would keep them off balance until the planet's Shields were raised, at which point 8472 would need to either retreat, or face annihilation when the nearest Imperial vessel arrives in response to the threat. (Assuming that there wasn't already a garrison present at the target)
And if you propse that each attack simply charge suicidaly at the planet, ignoring enemy fire. The planet may perish, but so will they. The Empire has more planets than they do ships. Victory by simple attrition.
Do they have nanoprobes in Star Wars? Even with them god luck killing them in the few second window before Coruscant go's poof.
A half-minute would be ample time for automated weapons enplacements to engage a sudden hostile presence. And given that Borg weapons manage to occasionaly damage 8472's ships, Wars will have little problem simply whiping them out.
And a world like the Big C. would always be guarded by warships, what with being the
capital and all...
Oh, did I mention Federation Transphasic cloaks? They are totally invisible as well as being abl to pass through normal matter. In other words ships outfitted with it could fly into an ISd or the Death Star and drop off Species 8472, Borg drones, or Klingons. All of which could mop the deck with Jaffa or Stormtroopers.
Invisible to Federation and Romulan sensors, and only seen to move through inert matter. (Like an asteroid) Its performance against a Shield or Starship hull is unknown, and means of detecting cloaked ships exist in Star Wars. (They scan for the disturbances in space caused by the mass of the ship itself)