Often between pack members submission is a mutual thing, in play one will often submit to the other then later on the other one will get on its back. Licking lips isnt a total flat out submission but it is a submissive act, however depending on the body language often only dogs notice tells just exactly is ment by it, whether its an "I'm sorry please forgive me" lick or "lets be friends and play" lick. Dogs arent always going to treat alphas the same way they do others closer to their ranking. Slight differences to the meaning behind the lick that body language and energy make clear. Maybe a better way of putting it isnt so much being submissive but telling the other dog theyre not a threat and want to be on good terms. In this situation its not so much saying I submit, but that I'm here to be a friend and respect your place in the pack.