Guaranteed means that they will get you a vehicle eventually, or they will pay your out-of-pocket costs if you have to rent something equivalent elsewhere. As an Exec Elite you would like to believe that your reservation is "more guaranteed" than the other folks you alluded to, but the reality is that everyone's reservation is "guaranteed", which ultimately doesn't mean much if they don't manage their fleet well and end up running out of vehicles.

As an aside, the Exec Elite benefit of guaranteed availability when booking 24 hours or more in advance is basically just a guarantee that they will let you make a reservation if they are showing sold-out on the website. As far as honoring that reservation, see the paragraph above.

Lastly, sell-out situations happen to all companies to varying degrees. FWIW, EHI (Enterprise, National, Alamo) is by far the best of the bunch, with Avis-Budget not quite as good as EHI, and Hertz/Dollar/Thrifty being a (very) distant third in our extensive experience. Not sure how much comfort that is to you, but fleet failures are pretty rare in general with EHI. You probably just got unlucky here.