I just answered the OP question of why a pusher is considered to be less efficient than a tractor setup. Does that mean a pusher wont work in this application, of course not.

Still it is a very ambitious project with lots of technical issues that need to be addressed, as well as the paperwork needed to operate a UAV in the NAS.
I think it is interesting that they are going to the extent of a purpose built airframe to collect their samples. I suspect that the speed of a full scale airframe may damage the samples they are looking to collect and hence that is the reason for this approach.