Jeremy Renner also references Avengers: Infinity War in his post, but his Hawkeye didn’t play a role in Infinity War, so his tease would more likely be about the upcoming installment.
Another angle is the promotion could be a “thank you” video of some sort for Marvel fans for hanging in there and showing up for the summer blockbuster that has hit over $2 billion worldwide at the box office.
It’s also important to note that the actor isn’t dressed up in character, so the promotion he is doing is as himself, but it definitely has to be for Marvel.