However, the pre-FC Borg interest in assimilating life forms seemed more a means to gain the technology of life forms than as an end in itself like it began to seem (to me, at least) with the post-FC Borg. Also, assimilation didn't seem to be the pre-FC Borg's main goal in the same way assimilation seemed to be the post-FC Borg's goal.
Assimilation is beside the point. The point is that the Borg are themselves half-organic. They are humanoids with mechanical parts grafted on. Their name
is short for "cyborg." Their own consciousness consists of organic, living brains interfaced with technology. Therefore, they are aware that carbon-based organisms -- specifically themselves
-- are life forms. That's got nothing whatsoever to do with whether or not they assimilate other species. It's about their own nature as a part-biological species.