Match Pose will only work for animations retargeted on to characters rigged in Blender using Uefy.
For example you will not be able to export the original mannequin animations directly to Blender. You will need a mannequin rigged in Blender as shown in https://youtu.be/8Ad4v7x41RA . All animations retargeted on that Blender rigged mannequin will be usable.
Once your modified/new animation is back in the engine you can retarget it back the original mannequin if you need to.