That’s a long story actually. Analogue has a (poorly implemented, in my opinion) marketing campaign advertising that their FPGA consoles don’t use normal software emulators like say SNES9x. Their devices instead emulate the consoles on a special chip called an FPGA that lets you mimic the circuits that make up the CPU and other parts. It’s still emulation, just a very different kind than what you normally get on devices like this. That’s also why it’s so hard to release new cores for the Pocket, because you have to create this mimic circuit design that often doesn’t exist elsewhere.
Analogue also doesn’t advertise the ROM playing nature, but their official cores are just a download away in a dedicated area they put aside for community contributions.