If you read the other retailers who also cancelled their preorders, you will learn that the shippment that did not arrive was a Distributors large order who supplies these individual retailers. Those who have been already selling the same mags have imported them on their own and likely not sought RCMP approval like this large distributor did or they would have gotten the same answer as the Distributor did.
It could even be a case of the sheer size of the distributors order caught RCMP attention and the other smaller ones flew in under the radar. In any case, nobody has posted any correspondence with the RCMP-SFSS other than Armtac and that email says they have not seen any 50 Beowulf mags that are not prohibited.