They chose not to toll his contract. As soon as a player officially files his retirement papers, his contract is void. He's essentially a UFA that no team is allowed to submit a contract offer to.
Either way, the Nabokov/O'Reilly rule is in play here - the second a player without an NHL contract chooses to sign in a foreign league, if they wish to return to the NHL they must pass through waivers before they join the team who signed them.
If Kovy plays this year in the KHL, and then signed a contract with New Jersey in in 2015, he would have to pass through waivers before joining the team. Radulov was different because he left the Predators with years owed. If Korolyuk wanted to return, the Devils own his rights. But Kovalchuk has no valid NHL contract and thus would be subject to waivers.