The building itself is what gets classed as "multi-family." A condo that is part of the building, but accessed from outside is still considered part of the building.
The ground floor/walk-out units here don't really have much to do with accessibility as they do with way of life. As you can see in the render in the Globe article, the building's main entrance lobby is located at ground level right next to these walk-out units and will feature access to the building's elevators. The whole building will be accessible. The walk-out units are just a feature/type of unit. If for whatever reason, you need to raise the main entry lobby up from ground level and can't adequately supply a ramp, wheelchair lift (avoided like the plague by architects because they're ugly), or elevator, then ground level/walk-out units are used to satisfy EO/ADA housing requirements.