so ... does this change mean that starbases are weaker now? because I have no clue how they worked before, in particular how many, if any, targets a bank can attack and whether the unused dmg could add to more than 100%.
It's going to depend on whether they adjusted the damage ratings of affected weapon banks to compensate, and we just don't know yet if they did. I'm betting they did; I wouldn't be suprised if they reduced the "base" (per-target) damage for any bank capable of targeting multiple foes. For instance, take three possible weapon banks that we want to be hypothetically balanced:
Bank A does 100 damage, but only to a single target. Bank B does 75 damage, max of 2 targets. Bank C does 50 damage, max of 4 targets.
Under the old system, there would have been no way to do this. Bank C would have always been stronger than A, because if there was only a single target it'd take 4x50=200. Under the new system, though, each has its own advantages; you'd want a bank like A if you're a capship focusing fire on a single target (other capships, starbases, etc.), while you'd want C if you were a starbase or wanted to wipe groups of frigates quickly. While they didn't say that they changed weapon damages, it could easily be viewed as an obvious consequence of the announced change; so, if a starbase's weapon banks did 200 before this patch, they could be dropped to 100 in this patch and STILL come out ahead in terms of effective firepower.
Also, realize that this change affects other ships as well; even if they don't adjust the damage ratings, then while starbases WOULD get stronger, so would the capships trying to kill them; they wouldn't kill the starbase itself any faster, but they'd be able to destroy any other defenders that came to the starbase's assistance. The losers in this scenario would be the frigates and cruisers that don't have multi-target weapon banks, which might not be a bad thing.