It's hard to know what the two companies were saying. Perhaps IMR was going to help pay for some of the cost for PGI. Perhaps Topps had stepped in and asked the lawyers for PGI to take the lead on it. There are innumerable things that could be going on that we will probably never know.
It's interesting that so many people though HG was winning because there was a settlement in process. As we see, it turns out HG was probably losing and knew it and in an attempt to avoid getting crushed by a counter claim or to damage other possible claims, they got out.
It's possible PGI made sure IMR was covered out of niceness. Could also be plans to assist one another with products. (MWO images in Catalyst books? No. PGI using Catalyst images in the upcoming Mechwarrior 5, a lot more likely)
I hope we see a statement from Catalyst at some point acknowledging what happened and discussing when the classics will be restarted.