Catalyzes the reversible retro-aldol cleavage of 2-keto-3-deoxy-L-rhamnonate (KDR) to pyruvate and lactaldehyde. 2-dehydro-3-deoxy-L-rhamnonate = (S)-lactaldehyde + pyruvate Binds 1 Mg(2+) ion per subunit. Homohexamer. Belongs to the HpcH/HpaI aldolase family. KDR aldolase subfamily.