OK so a control rig is a type of exoskeleton that can be mapped over a skeleton that does not use the required naming convention. Usually using a relationship constraint that will allow for “any and all” rigs that does not use the preferred naming convention to be used. A good example of practical use would be Motionbuilders control rig as to how it can be set up.
At the moment Epic’s control rig, as an opinion, is yet fully completed as to intended design in Unreal 5 except for possibly animating “some” keyframes but is rather limited as to what it could do as to removing the need to retarget Epic animations to a custom rig, such as Genesis 3/8, as well converting ready to use BP’s.
IK or Inverse Kinetics is an element that could be added to a control rig or used standalone as via 2 bone IK where the end effector can be attached to an object that drives the attached joints in an FK type direction.
Over all I would have to say that Epic is following a path towards full body IK which has been around for years so yeah IK would be required