In GLA13 (GLAH13), values of "surfType" all indicate combinations of surface types, with no values for pure sea ice or pure land. Is this correct?

Yes, surface type flags are never purely sea ice; ocean is always combined with sea ice. Since GLA13 is a sea ice product, if the sea ice bit is not turned on in "surfType," it will not be in the product. This explains why there are no "pure land" returns. Also, the mask resolution is sufficient to flag many values as multiple surface types.