When I was sitting at my computer for long periods I'd just make a warm bottle with hot water to put my feet on and also drink hot tea. That was enough to be comfortable even at around 16°C.

Maybe we mean something different with "jacket". Don't people wear sweaters and thermal underwear at home? And with those you would be conformable at 15 to 17.

English is not my first language. I suppose you would say "sweater" (but I said jacket because I prefer those garments with a zipper at the front: which I thought was always called "jacket" regardless of whether it's very light or thick).

You're not wrong. What people think will depend on context, and probably on region. In US English, jacket is a wide ranging term that could easily include a zip up sweater. If you wanted to be extra clear, you could saw "sweater jacket" or "fleece jacket" (if it's not woven).

It is very difficult to compare self-reported temperatures! Really if you get into it, there are many crappy sensors, crappy thermostats, thermostats mounted near single pane glass, etc... I spend some time complaining about my Nest V3, many people did, but it's just too difficult to get a reliable measurement. Btw, in the end my conclusion was that the Nest was doing quite well. Just the Shelly and the Hue sensors were easily 1-2 degrees off.

Btw, what's up with Nest? I want another V3 in my new house, but they are not available anymore??