The only ones saying anything definitive about what the Borg were doing in Q Who? were Q and Guinan. The Borg themselves never stated whether they were only after their tech or not.
Well... The first Borg drone(s) only analyze the ship's computers and completely ignore the crew (in following encounters the Borg beam aboard and try and to attack/assimilate/capture.) Their only interest seems to be in technology and they only threaten the crew with punishment if they resist an attack. No "biological and technological distinctiveness to our own" statement. The intent of the Borg changed between Q-Who and BOBW, the crew even points this out in BOBW.
The crew were only saying "I thought they didn't do this", it's all based on their own assumptions. They have no proof because they have no real knowledge of the Borg.
Presumably once Picard was de-assimilated he learned the whole story about how they do assimilate people.