It's using a separate MC34063 boost generator. Despite what everyone says, they work well as long as you take care to add a circuit to flush the gate on switch off. The 34063 be default is an open collector drive, and this makes the discharge of the MOSFET gate slow and so the device gets hot. If you add a discharge circuit, it stays nice and cool.
The other thing about this design is that there is a level shifter between the Wemos and the HV chip. Running from a Wemos at 3V3 to the HV chip (which the spec says needs to be run from > 10.5V) just wasn't right, although it seems to work well enough in the short term. those are the 4 TO-92s near the Wemos.
Ian: understood about the level shifting. I always have used a 4504 IC to shift when using the HV line of chips but I suppose transistors are fin too. I remember Jeff telling me his Nixichron avoided shifters by using an Atmel controller (5v outputs) and running the HVs with out-of-spec Vcc (low, 10V or something) which apparently works too but seems a bit shaky to me.
Hartmut: I'm not sure how quickly I can get started on any instructions but let me take a look at what you have so far. Do you have any kits of parts ready to go? I'm asking that because I think I would need to get/buy/whatever a kit to build here for purpose of taking pictures of every construction step.
Look into it later when the dust is clearing off the crater.