So ... uh ... When did Brainiac become a Borg?
Brainiac has been many things
over the course of his existence. He was originally just a bald green alien with a pink shirt and a pet monkey. Then he was retconned into an android (to avoid legal conflict with a "build a computer" product of the same name
by making the character a weird sort of tie-in to the product), though he still looked largely the same. In the '80s, he was revamped into a more blatantly robotic form, perhaps influenced by The Terminator
. Then in the John Byrne reboot, he became a green-skinned alien again, and used telepathy to take over the mind of a human circus performer, of all things, in order to wreak havoc on Earth. Then the human Brainiac had a duplicate of his alien body manufactured, but it was still organic.
But then Superman: TAS
introduced an impressive new version of the robotic Brainiac, who was a clear influence on the Brainiac in this treatment: he was responsible for the death of Kryptonian civilization (by concealing the fact of the planet's imminent doom), he could download his consciousness into duplicate bodies, and in some stories he had multiple duplicate bodies in reserve.
So the comics followed suit and had Brainiac switch to a robotic body, which was then changed into a body made of countless nanobots. Then, more recently, Geoff Johns introduced a version that amalgamated pretty much all the earlier versions, introducing a "real" humanoid-alien Brainiac of whom the others had simply been created avatars.
And then there's the Smallville
version which is the "Brain InterActive Construct," again going with the "rogue Kryptonian AI" version from S:TAS, but functioning more like the T-1000 Terminator, a shapechanging "liquid metal" entity.