In fact, the website says that each townhome (yellow on sales map above) offer two private spots, and the carriage homes (green) offer one spot; it's tucked under an accessible from the allies and small parking areas. The four flats on the edge of the property will all have, I believe, some of their parking underground parking. 124 Alfred, which is the one soon to open, has all of its parking underground.
In any case, the entirety of Brush Park just about is zoned PD (Planned Development), so the normal requirements don't apply. Most of the other developments planned for the neighborhood have underground parking. Check out these plans for the rest of the neighborhood:
Brush + Watson
112 Edmund, 2827 John R & 105 Alfred
SoMA Brush Park