They knew that the assaults occurred (DOH!) so there was proof of that. What they didn't know was who the perpetrator(s) were. Why the first statement wasn't joint may have more to do with Iowa wanting to get right on it while Pollard was dragging his feet - I can only guess about that. But generally the faster you get info like this to the public, the better chance you have of getting someone to remember.
So if they were trying to jog peoples' memories or get someone to come forward, why was the initial press release not clear in what happened? Why would they say there was an incident and be very vague about it, and why didn't they ask anyone to come forward directly in the initial press release? All it said is "We are aware of an incident, we contacted Iowa St., and we're investigating." Nothing about that tells anyone anything about what happened, let alone would prompt someone to logically think that something they saw needs to be reported.
Again, the initial press release was terrible both from a PR standpoint and as a tool if they were actually trying to get more info. It did absolutely nothing except create a story that no one knew about and offered absolutely no substance, only to be backtracked 3 days later in a press release that should have happened first.