It would be rather silly for Discord to be able to use the elements of harmony, since they are what locked him in stone the first time. However, consider. Discord was meant to be in concerto with his Sisters from the beginning of time before he fell. So now that he's had a Heel-Face Turn, he's now worthy to have elements of harmony in concerto. Kinda a final show that he has been redeemed for the alicorns.