Not sure of the exact short story he's referring to, but it must take place in his Laundry Files series https://en.m.wikipedia.org/wiki/The_Laundry_Files https

Not sure of the exact short story he's referring to, but it must take place in his Laundry Files series https://en.m.wikipedia.org/wiki/The_Laundry_Files https://en.m.wikipedia.org/wiki/The_Laundry_Files He also has another series which he just finished that is very good as well about a family with the ability to step between diffrent timelines that's an interesting deconstruction of the trope. https://en.m.wikipedia.org/wiki/The_Merchant_Princes https://en.m.wikipedia.org/wiki/The_Merchant_Princes ********Spoilers******* . . . While it starts as starts as a fantasy series, it eventually comes out their magic abilities are remnants of advanced nanotech from an interdimensional empire. Apparently a publisher he didn't like working with had the rights to his next Scifi series, so he had to start it as fantasy to get around the contract.