It is entirely possible. On trans-continental flights Delta have increasingly been holding back a higher number of seats for day-of-departure upgrades (likely in hopes of being able to sell more F tickets).

That said, i really think that if you want to use EF, you should pay for it (or try the free trial, or one of the many alternatives, such as the fake booking you already tried). It's a service that charges for money, and as someone who makes software for a living I am not going to help someone rip off a fellow software engineer.