Microsoft's parity clause forces indie developers who do not have the resources to make a multiplatform game from the get-go to release on the Xbox first because then they can release to the PlayStation at a later date if it does well, knowing Sony do not have the same parity clause.
I personally think Sony have the right to advertise this fact, considering that if they did have a parity clause most developers would release to the PS4 first due to the larger install base and then that game would never release on the Xbox One due to Microsoft's parity clause.
That does not stop a company from releasing on the ps4 first and then the x1. The clause does not bar you from already being out on another console as far as I have seen. It refers directly to a game that is not out anywhere yet. Plus, considering MS has said they will work with any dev that cannot agree to that clause, maybe MS has smartened up enough to know that any rules they make cannot be applied universally.
So basically, if a developer cannot afford to be out on both platforms at once, they have three choices:
1. Release on the X1 first
2. Release on the ps4 first
3. Release on both at the same time.
The clause does not make any of those options impossible.
Still, as you point out, Sony has the right to bring up the fact that they do not lay down any rules for their service. There are literally no limitations. So if MS has any rules at all, Sony can use that against them as they are here. Its a very effective strategy online. People eat this stuff up. Then it trickles down to the general gamer masses as those people online spread things around. Its cheap but effective pr.