Hi Alice,
Honestly, it's not clear to me where these drivers came from fully. However, the descriptor of the chip is a generated file name LPC15xx.h (included). On the top of this file, it mentions that it was created in 2012 using SFDGen. I belive these might be of CMSIS drivers on Keil that I could find but maybe an older version. I am not sure if it was copied over exactly from here.
The drivers/API for the peripherals are in the similar style as those of the LPCOpen in terms of functionality with some clear modifications that have been made by some previous developers that worked on this code.
Best regards,
SG