Murahilin, most books describe longkong as the same species as langsat. I've seen that Dr. Salakpetch, a fruit specialist in Thailand describes it as a different species. In either case the fruit is quite different from langsat. ( (Cabbage, brussels sprouts, kale, broccoli, and collard greens are botanically same species, Brassica oleraceae, but we wouldn't interchange their names on a plant label, would we?)
Harry, if the fruit you tasted had a grapefruit taste it was probably langstat.
To complicate things a bit further there are 4 distinct versions of this fruit:
Langsat, Duku, Duku-Langsat, and Longkong.
The longkong appeared on the scene last i believe. It usually costs about 3X as much in markets in Thailand as the langsat. So that will give you some idea of the improvement in quality.
Longkong can fruit in 6-8 years in tropics. Langsat takes 12-20 depending on treatment. I usually have seeds of langsat and longkong in late summmer. Longkong seeds are much more expensive because most fruits are seedless.
Oscar