Someone doesn't understand how a threat works. The threat of taking Kharg is leverage. Because your logic works both ways: if Iran keeps the straits closed, what incentive do we have to not take Kharg?
Furthermore, if we were to take Kharg, Iran only has no incentive to open the strait IF you assume that Kharg is lost to them forever. Remove that assumption, and Kharg becomes a bargaining chip, and a valuable one at that.
Furthermore, the loss of Kharg makes it harder for Iran to keep the straits closed. The IRCG isn't actually a charity. Its members don't work for free. They have to be paid. And when your ability to keep paying them (including expectations of future pay) gets undercut, your ability to keep commanding them does too.