There's no leverage gained by players going overseas. From the owner's perspective they can't be profitable under the current system. If NBA superstars go overseas or not, that doesn't have any impact on the owner's stance. They're not going to loose more money just to prevent stars going abroad; that makes no sense. A superstar player is actually risking a lot more by signing another contract; a serious injury that could void their current contract or serverely limit their next one if they're at free agency.
Of course there is leverage gained by the players playing overseas. The owner's leverage is that they can lock out the players, stop paying them and since they are much richer and have money coming in from television contracts, they can outlast the players because the players are less financially secure and will need the paychecks that they receive.
But if the players can make money through endorsements and playing overseas during a period where the owners think they have shut off their access to making money, then they aren't forced to quickly get back to the bargaining table and give in to the owners because they need their money. How much leverage this gives the players and their union as a whole is debatable and yet to be fully realized but it is still leverage.
Its all a game of financial chicken to see who flinches first and is forced to come to the bargaining table and gives in to the other. If the season is delayed or called off and a large portion of the membership sees they can still sustain themselves financially, there's no immediate need to run back to give into the owners demands.