When is that "established"? Certainly not in that episode--nor prior to "Q Who?". I don't recall that it was unequivocally determined to have been the Borg at all (though I could be wrong--I've not seen TNG era episodes as often as TOS, so I don't recall all their details) and even IF it has been, I'd bet it was long enough after the episode for either A) it being another example of inconsistency (among many) in Trek or B) a post-FC episode of Voyager in which the assertion is part of the ret-con established by FC.
In the solar system from "Q Who?" where the Enterprise-D meets a Borg cube for the first time there was also a devastated class-M planet... and Data remarks that those devastations match with those from the Federation outposts which were destroyed the year before.
So unless you want to argue that a) Data was either mistaken or b) the Borg cube was just in that system by coincidence and had nothing to do with the destructions on this planet, the Borg were in fact responsible for the attacks on the Federation and Romulan outposts along the Neutral Zone.
I don't have a transcript for that episode, but the Memory Alpha article on "Q Who?" states:
A survey of the only Class M planet in the system reveals that while there was once a civilization there, it has been ripped away from the planet, "identical to what happened to the outposts along the Neutral Zone." A cube-shaped ship then approaches the Enterprise, and scans of the ship show nothing.
They episodes don't contradict of course, behold:
Cube arrives in Alpha Quadrant, assimilates a few outposts. Cube leaves; there's nothing interesting here. The Federation and Romulans are too primitive to bother with. Cube leaves.
Q sends Enterprise 7,000 lightyears onward in a moment. Cube sees. Borg go: "Wtf!? We just were with these guys. They shouldn't be able to be here. They're primitive morons not worthy of assimilation! Better check this ship out."
"Yup, these guys are nothing as we knew, so how did they get here? And do they really think they can outrun us?"
Q arrives, and zip.
Borg: "What the hell!? There they go again! All right, that's it, no more Mr. Nice Borg; we assimilate the whole bloody bunch of 'em and be done wih it. That way we will certainly know what happened, and posses whatever technology just did that, if it is indeed technology."
So yes, if Q had not made the Federation enticing, it would have taken much longer before the Federation actually confronted the Borg.