Let's be honest: Broforce was basically already an unofficial Expendables video game, so it's not a huge shock to hear that they're making it official. It is, however, a reason for there to be more (free!) Broforce, which is always a good thing.
Titled The Expendabros, the promotional tie-in is a collaboration between the people who made The Expendables 3 and the developers of Broforce. It features characters from the movie leaping and exploding their way through ten Broforce-style missions.
It's free to download on Steam, and if you like it, I highly recommend getting the actual early access version of Broforce, also on Steam. It features a lot of the same actor-bros as The Expendables, and it owns.