I have noticed that unfortunatly the provided code example will not work on newer AVR devices over the same, large temperature range. Compare the pull-up resistor current graphs of the ATTINY13A (upper) to the ATTINY202 (lower):

On the ATTINY13A the resistance value increases by about 5% for extreme temperatures (either -40 or 85°C). On the ATTNY202 the resistance can vary between 24k and 32k nominal or about +10/-20%!

This means if you want to use this code reliably on newer parts, you have to:

1. Re-calibrate the internal pullup resistors whenever the temperature changes or
2. Use an external pullup resistor (which can be switched)