Runaway greenhouse effects on other planets have resulted in surface temps of 400C+

Yes, but Venus has 93 times the atmospheric pressure of Earth, and it's overwhelmingly CO2.

Earth isn't likely to get a true runaway like that until the sun gets another billion or so years on the clock, and when it does it will be water vapour as the oceans are promoted to atmosphere.

What we're doing to ourselves is still bad, of course, but it's nowhere near that bad.

Is there a possible scenario for that on earth, that results in every place on earth having a temperature non survivable by humans?