Yeah, there's been many hints. This one being the most obvious one. If you read the comments, they are all "OMG SHENMUE 4", and seeing numerous big website talking about a Shenmue 4 tease, that'd be pretty bad from them if they aren't working on a Shenmue game or even a Yu Suzuki's project. Like really really bad, especially for a young company that have no fan base since they haven't released anything yet.
So yeah, while I'm still not 100% sur that it's Shenmue 4, I'm pretty confident that it's going to be Shenmue related. If not, then they really need to be clearer when communicating about future projects.