Indeed, there is a garage. The PDF seems to be glitchy so the screenshots are a bit messed up. Looks like one level below grade. Not sure if I'd want my Range Rover 15 feet under Seaport Blvd, but...I don't have a Range Rover.
Anecdotally, pretty sure I saw two roll-up garage doors right on Seaport but I don't recall if there are obvious curb cuts or if the whole sidewalk fades to street level around that area. The second picture below makes it clear there are two car elevators available, so that's likely what's happening here.
April 19, 2016 Public Meeting Presentation
bpda.app.box.com