But also, if they did want to do that, they could probably backdoor it pretty easily just by patching the binary a bit. Reverse engineering and changing binaries is way easier than cracking encryption, and they probably have some of the world's top reverse engineers.
The fact that they asked for an existing algorithm backdoor, among other things, makes me think they really were trying to crack at least one thing encrypted with it. As for whether it was to deal with a time-critical situation or whether that was just a confidence trick to make him more likely to give up the code, who knows.