I think the reason people consider it a "strength" program is that the basic idea behind the % loading of 5s, 3s, 5/3/1s followed by a deload is designed to enable longterm progress / weight increases on the main lifts. Which is obviously a good thing, in many ways, but goes against the common idea of "building muscle" in 4 or 8 or 12 weeks. Of course, BBB is specifically designed as a hypertrophy / mass gain protocol, and includes a lot of additional relatively low-intensity volume for that purpose.
I've never done BBB myself but would like to at some point, and consider it a good recommendation for a lot of people looking for a new routine. Perhaps including the OP, if his goal is mass.