In step 7 (plug in AC with battery and RTC disconnected), the leds indeed blinked red a few times as documented. However, they switched to green quickly, which I think means the BIOS is doing a RAM calibration (i.e. the machine has turned on). I suspected this was because I have power on AC attach enabled in the BIOS, making me believe this guide was not written with that setting in mind (however, at the end my BIOS settings were reset, suggesting that BIOS settings are stored using the RTC battery, implying that my original settings were already gone at that point). In any case, I did not want my machine to boot up at this point, so I unplugged AC when the leds were still green. I plugged in again to confirm it still blinked red and unplugged quickly (while still blinking red).
I then continued with the procedure (not waiting another 2min after unplugging AC, since the instructions did not say so). At step 10, after plugging the RTC and battery back in, I connected AC again - but there were no led blinking leds (no power-on either). Following the instructions for this case - unplug and replug battery still showed no red blinking leds (should I have waited 2 minutes again?).
So I went back a little - unplug battery (kept RTC inserted because it is fragile), wait 2min, plug AC, saw red leds, quickly unplugged AC, waited another 2min, reconnect battery, plug in AC and yay - red leds as instructed.
Again, the red leds turned green and the laptop powered on. I left it, and the leds switched between red and green once or twice IIRC and eventually it started booting linux. Since it was still disassembled (instructions say to leave it charging while diassembled for 5min before disconnecting AC and assembling), I forced the power off with the power button, waited for 5 minutes and continued with the instructions, with no further surprises.