It was a contract dispute. It was about who was appointed to do what and what they said. And it is hard to prove a negative: ie. the ecat information given to IH did not allow them to make a working ecat. It's even harder to convince a lay jury of such things because they have no scientific background, in general, and do not understand the facts. And the risk was bankrupting his company. Darden had plenty to fear.
Rossi denounced Darden, so Rossi had to prove his reasons. Darden would have find himself in that situation only in relation to the countersuit. I do not think Darden was so sure of the evidence he had, but above all I do not think he was afraid of bankruptcy, since by now it seems to have become his hobby.