I am interested in finding out if there is any issue with placing components on the bottom side of a PCB under the IMP002 module. I know the antenna part of the module should have nothing under it but what about the rest of the module?
No, that’s ok, but bear in mind the keep-out area: you shouldn’t place vias (ideally, even tented ones) under the exclusion area noted in the datasheet.
Also, if anything is RF-noisy, then putting it close to the antenna is a bad idea.