Catalyzes the NAD-dependent oxidation and subsequent decarboxylation of D-threonate 4-phosphate to produce dihydroxyacetone phosphate (DHAP). 4-O-phospho-D-threonate + NAD(+) = CO2 + dihydroxyacetone phosphate + NADH Binds 1 divalent metal cation per subunit. Homodimer. Belongs to the PdxA family. PdxA2 subfamily.