@richard got a couple of W1209 boards, and some of them just couldn't be flashed. It turns out the at least one manufacturer (namely TENSTAR ROBOTS) makes boards with a nearly pin compatible 8051 based µC (a Nuvoton chip in a TSSOP20 package). Details are at the end of the GitHub Issue here.

Mitigation: don't buy boards that look like one of these:

Note that C5, the Vcc capacitor next to the µC, is unpopulated. The backside of the black board is bears the label "TENSTAR ROBOT W1209". The label on the back of the green board is "XH-W1209". The Nuvoton is quite fast, and if you like programming the 8051 it's maybe not the worst choice. However, please don't count on my help for programming it. I'm done with MCS51 ;-)

@BigVulcanDeal let me know that there are boards with a 5k, instead of 20k, reference resistor. That's a good match for measuring temperatures in the range of 30°C to 50°C, but you'll need to replace the linearization table in measure.fs.