I'm pretty sure the NORAD folks (or whatever they're called now) would have been aware of the potential collision, and issued a warning to Iridium -- I think that's pretty standard, but it may require Iridium to "subscribe."
In any case, if Iridium got a warning, they had a couple of options: do nothing, or do something. The decision would be based on three things: how soon will the event occur (is there time to do a maneuver?); and how well-known is the orbit of the Russian-made object?; and how soon can NORAD compute a more precise orbit solution?
The latter situation is not that simple to address: the standard NORAD orbit solutions are necessarily fairly inaccurate -- it's a matter of getting sufficient tracking data for the object, in an enviroment where there are 8000+ other bodies competing for measurements.
So it's only when there's a potential close approach that the dedicated tracks and more precise orbit determination begins. Even so, the uncertainties can be significant.
So, again, Iridium has to base their decision to burn, on an assessment of the uncertainty in the NORAD orbit solution: do you go with the "big space" argument, and hope for a miss; or if you're not sure about the NORAD orbit, will doing a burn maybe make things worse rather than better? Or are you going to do a burn and get well away -- which means downtime and perhaps complicated commanding if the burn direction is retrograde (a real issue for Iridium).
After all that ... my guess is that Iridium knew about the close approach and decided to do nothing, in hopes that the collision would not occur.
999 times out of 1000, they'd have been right.
Well, remember that all it takes is a few low orbit rockets and about 2 tons of junk with an explosive and you can have a mini-Kessler syndrome, if you do it right...
I’m surprised some govt’s havent though of the idea of shutting off space access this way.