Whatever it is, it looks like you flash it via JTAG, note the TDI TMS TCK pins in the bottom left corner. And you’d need an isolated programmer since all that sits on the scarey side of the barrier.
I noticed they used an STM32 with an ST-LINK header in one of their products:
https://www.webx.dk/oz2cpu/energy-meter/energy-meter.htm
That too would need an isolated STLINK programmer. Actually, that entire hack project is very dangerous (they do mention it in the fine print). They’re wiring up external hardware to the live side of the barrier so all of the display hardware is live.
Now, speaking of STM32s… what’s been happening out there in OEM / STM32 land?