It sounds like they are looking for their first employee, not a cofounder, but they don't want to pay you a salary.
It also sounds like they haven't done any validation on the idea. An idea that sounds nice is worthless if there's no market for it.
From the short summary it sounds like two dudes are trying to find someone to build their idea because it will make them rich, there will be lots of conflicts with the founders, no sales, and a protracted fight at the end over who gets to keep the worthless IP, and you'll pay more in lawyer fees than the company ever made in revenue.
It also sounds like they haven't done any validation on the idea. An idea that sounds nice is worthless if there's no market for it.
From the short summary it sounds like two dudes are trying to find someone to build their idea because it will make them rich, there will be lots of conflicts with the founders, no sales, and a protracted fight at the end over who gets to keep the worthless IP, and you'll pay more in lawyer fees than the company ever made in revenue.