The new device is called FP-GBC, not FPGA.

FPGA (field-programmable gate array) is a technology where the chip contains all kinds of circuits, which can be programmatically "rewired" in specific ways to make it into a fast device-specific circuit, such as an emulator of the Game Boy GPU that can drive a modern LCD without needing to interrupt a CPU. Its function can be exactly the same as the original but way cheaper than manufacturing a new silicon die for the small production. More complex FPGAs could also emulate the CPU, sound and everything else from a GameBoy, or even handle intensive tasks like crypto mining with relatively good power efficiency. Both the Analogue and FunnyPlaying GB replicas use FPGAs.