It must have backup NiCd rechargeable batteries. No capacitor from 1970 that can fit in that clock could hold enough charge to drive a nixie display for 25 minutes. You'd need a pile of modern 1 farad supercaps, or a Maxwell Boostcap or something like that. I'm guessing it's a bad English translation.
It's a little pricey for a Nixie clock, but it's not just Nixie clock guys bidding on it. $850 isn't too bad when you're wearing a $3000 Longines wristwatch. It's entirely possible that the winning bidder doesn't even really know what a Nixie tube is.
Are there any other watch collectors here? I've paid much more than $850 for various watches over the years. It's a far uglier habit than collecting Nixie tubes.