Both Catalogue of Life and WFO consider them synonyms. Now why B. cochinchinensis is selected as correct name:
The species was first described as
Toluifera cochinchinensis by
João de Loureiro in 1790, it was transferred to Buchanania and named B. cochinchinensis (Lour.) Almeida in 1996. On the other hand B. lanzan Spreng. was first described in 1800, so gets relegated to synonymy on basis of Principle of Priority.
At the same time Kew Database POWO, treats two as distinct belonging to different genera Buchanania lanzan Spreng. and Glycosmis cochinchinensis (Lour.) Pierre & Engl. You can choose whom to follow.