I know he said that but I've seen other F1 drivers make certain claims about their contracts etc. that have turned out to be incorrect and driver contracts are said to be very complicated and almost unreadable to a non-legally trained observer.
I believe Rosberg leaving abruptly caught a lot of people out and while Ricciardo might not have an explicit if Mercedes/Ferrari have a vacancy clause I would be very surprised if he couldn't make himself available if the situation arose.
I also believe that when he was asked about a clause he'd shut down the talk immediately. There's no vacancy yet and might not be, he's a Renault driver and anything less than an affirmative denial of a clause will just allow the rumours to run on.
Not sure why you wish to continue flogging this poor horse but all contracts are logged with the contract board and have real teeth.
Normally, only retirement can get a driver out of a contract.
Usually, such contracts will have a predetermined penalty fee if a driver wants to leave. Under EU law, you can't force anyone to work but you also can't just up and leave to go work somewhere else. So either a lengthy gardening leave or a penalty fee.
As to Ricciardo's case, obviously outsiders don't know all the details but considering Renault was looking for a star to lead their team and willing to pay crazy money for a Ricciardo desperate to get away from Verstappen, they surely weren't interested in allowing their expensive star to seek new pastures while they are busy building the team around him. That's partly why he got so much money without really justifying it with his results IMO.
So if a slot opens up, Ricciardo could go there if he or his new team were willing to pay what I presume would be a hefty fee. Even Williams got a lot of money out of Mercedes for Bottas.....