Complex sentence: Rather, it says that the compound's stereochemistry is related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde--the dextrorotatory isomer of glyceraldehyde is, in fact, the isomer.
Split into simpler sentences: Rather, it indicates the compound's stereochemistry relative to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde. The dextrorotatory isomer of glyceraldehyde is, in fact, the isomer.