The only way they could do this concievably is though mining the strait, US 5th fleet might have something to say about that, and its not likely Saudi Arabia is going to be happy with it either. AQ is going to have a hard time making good on this threat.
Al Qaeda is going to have an easier time than me here think. Al Qaeda has sooooooo many ways that they can do this and limit the US and Coalition naval forces ability to prevent such, as well. Nonetheless, til al Qaeda backs up their threat, it all amounts to rhetoric.
I don’t think they can do much or they’d act first and boast about it after.
Probably more an attempt to have some fresh jihad propaganda to distract from what’s happening to them elsewhere.
No doubt they may sometimes attack a ship or lay some mines (if they can acquire enough to matter), but they’d have to deploy a lot of continuous capability to be able to control that area.
Anyway, another reason to keep exterminating them, and maybe it will give some new motivation to Egypt, the Gulf States, and Europe to care about stomping out jihadists.