The holder of an IMCr will by holding the IMC rating be able to demonstrate that they meet the requirement of 15 hours dual instrument training.
That, Homeguard, is exactly how the FAA requirement for "authorised instructor" is met. If the instructor was not "authorised" for the training, you would not have got the IMC Rating in the first place
IMHO, the FAA "authorised instructor" requirement is intended to prevent instrument instruction by people not authorised to do it... fairly obvious I suppose.