We can't even say it perfectly emulates the CPU. It may pass all tests we know about, but even 1980s CPUs were complicated enough to have odd niche behavior.
It's some great hardware, but I think a lot of people have been hoodwinked into thinking FPGA = perfect. Often some of the same people who turn their nose up at software emulation for equally bad reasons.
It's still emulation. Yes, it's emulating hardware, as close as possible and often indistinguishably close, but it's still emulation.
For example, my EDGB X7 runs fine on any real Game Boy I have, but can't switch games on an Analogue Pocket.
Another weird issue that I had was that if I launched my Pokemon Crystal save on Pocket it would, for some reason, permanently change my character from a boy into a girl (without saving the game!). This wasn't happening on my Game Boys (I restored the save a couple of times to test it).
It is not emulation, it is hardware replication.
And yes it is not always perfect. As with any replicated or cloned hardware it is just as good as the available information and the skill of the manufacturer.
Not always, the mister would need more elements to do an actual 1:1 for many newer consoles and the cores are often reverse engineered best guesses and not replicating the original asic design.
On the other hand, original hardware goes through revisions and the silicon can change (snes 1chip vs 2chip for example) while still be perfectly compatible so it really depends ho much of a stickler you are.
By its very nature, an FPGA is not original hardware.
An FPGA is hardware that is designed to be very similar to original hardware, but it does not actually use original hardware components, and because of this it can actually have bugs or inaccuracies that were never present in original hardware.
It is FPGA based, due to this it can be configured on hardware level to exactly replicate the original hardware of the retro system.
This and that it runs directly and not through some emulation layer and modern OS and stuff means that it gets as close as original as it can be, with zero lag and delay.
Not only perceive, it is often multiple frames from multiple lag sources (input lag of the USB controller or even worse Bluetooth, display lag from the monitor, rendering lag from the emulator, framebuffer lag).
Playing fast paced games with frame perfect movement (Megaman on the NES for example) is so much harder on a emulator with all the lag, even on very recent hardware.