Right, the volcanos teroticaly made about a 4 degree difrence. Then that heat unacned underwater metahne, which lead to further climate change.
A question rather than a personal belief -
The impact and subsequent volcanicity would send a fair amount of material into the atmosphere.
With the sun blocked a nuclear winter results, reducing global temperatures.
Impact and World temperature rises say 5deg C (based on no research on my part)
5 years (or whatever) later world cools 10 degC
Continued eruptions increase by 0.1 deg C every 500 years
Clensing of atmosphere reduced the nuclear winter increase by 0.1 deg C every 100 years
Stable world, but with pollutants in atmosphere temperature some 5-10 deg C
Next 100,000 years...... World balances out.
It would not be a case that increased temperature happened, but there was no stable environment above sea level for several 100k years.
Just thinking out aloud.