Fun but not likely to be a big seller in the UK/Europe.
It is indeed an ARM based emulator inside a miniature recreation case, in the same vein as their recent C64 or A500 mini's.
It's intended as a plug and play way to nostalgicly play games of the era from Atari's 8 bit home computer lineup (and the ill-fated 5200 which utilised the same chipset as the micro computers rather than the VCS hardware branch used in the 2600 and later 7800).
It's bundled with 25 licenced classics, and the manufacturers (RetroGames Ltd) are smart enough to realise that these platforms are generally hacked immediately to add extra games, and instead of trying to block this (like nintendo etc did with their mini consoles) they embrace it and support sideloading via USB as they did with thier earlier devices - probably helping them sell more devices overall and keeping the nerdier end of the buyer spectrum a bit happier.
The 400 is maybe a bit of a wierd choice from a UK perspective, as they didn't sell many here (Spectrums, C64's, Amstrad CPC's, Dragons and BBC's/Electrons all comfortably outselling them) nor in the Europe, but they sold about 3m of the various Atari 8-bit's in the states, so there's probably a bigger nostalgia market for these mini machines over there.
Inerestingly, Retro Games' lawyers sent a legal C&D to a Polish company recently, who had announced a full size Atari 800XL "inspired" machine. So I wouldnt be surprised to see a full size Atari 800XL appear (similiar to the C64 Maxi) in due course running the same ARM guts and software as this 400.
What I (and I suspect many others) really want though is a bloody Amiga A500/A600/A1200 maxi, with a decent, fully working keyboard etc. I suspect we'd get an A600 or A1200 if we did, as they were a lot smaller than the massive Amiga 500.
Give this relationship with Atari (which doesnt overlap with neu-Atari's first party 2600+ and similiar devices) I wonder though if we'll see an AtariST Mini (or maxi).