Trump was probably serious....likely someone saw the movie and mentioned to Trump "How about using Alcatraz again?" and Trump thought "Great Idea!" and that is all the thought/preparation/planning that went into it.Reopening Alcatraz is a stupid idea. It would result in a smallish prison that is extremely expensive to operate. To even get it to operate would be very expensive and difficult to accomplish. The biggest drawback is how you get operating utilities on the island from fresh water to waste management.
I'd say it's just another frivolous distraction tossed out there by Trump for the Left to fight over.