Today I got to investigating, what really happened. Found out that despite me forcing the main power to the chip, core voltage (1.8 V) was still coming on for a brief moment after pressing power button. So I tried forcing that core voltage, externally. That didn't change anything, the chip still won't SPI. Then, I continued probing around, and there were quite a lot of signals coming on and off during that brief moment. It was clear that the chip is not quite dead yet, maybe just sleeping.

I tried SPI-ing the chip during that brief power-on period, and got something. That made it clear the chip WAS WORKING, it was just immediately powering off.

Then, after a bit more probing, I came across the pin that detects if charger is connected... Aha! I recalled, there is that demo mode, where it wouldn't power on if charger is not connected. So I plugged it in, and EUREKA! it works!

So, now it became clear, that my semi-broken flex connector caused it to misinterpret the key combo I was doing, and I enabled demo mode instead.

How embarrassing!

OK, back to hacking!

There remains a question. How does one force the chip on, to re-flash it for example, if the firmware powers it off immediately.