Boundary layer recording requires a rigid backing surface larger than half the wavelength of the lowest frequency of interest, so SASS is definitely not a boundary layer arrangement, despite appearances. The baffles are far too small for it to count as that.
It's more like a Jecklin disk or a stereo sphere arrangement--a closely spaced pair with some acoustically opaque body in between. (In German there's a single compound word for that type of recording--Trennkörperstereophonie--that's hard to translate into less than about seven words in English.)
--best regards