yes, lets ignore the likely 30 to 60 seconds you had where his ulti let you know he could see you, and therefore you had 30 to 60 seconds to run away.
The reaction time is him activating his ult. You have more than enough time to react to him activating his ult and him assassinating you
If I was 0/20 vs a 30/1 rengar, I'd die in 0.3 seconds too...
thanks god rengar can still assassinate people with a ful ad build