First off, this is not 3 seconds. If you look at the shot clock, it's barely 3 full seconds. It goes from 9-6 which isn't necessarily indicative of 3 full seconds, and you can see the defender begin to slide out as it approaches 3 full seconds.
Also many real defenses, even in playoff games have players they treat as non-factors from 3. Ben Simmons has shown to be one of those players attempting only 3 three pointers in his last full season of basketball while making one. He's probably rated in the high 50s at best as a 3 point shooter and can afford to be ignored on the perimeter. Tony Allen and Roberson come to mind as guys who have been completely ignored by defenses when they are behind the line.
How the Warriors started ignoring Tony Allen and completely shut down Memphis' offense
Here is a full video of literally no one guarding Roberson
I get the concern, but to me this clip doesn't heighten those concerns. I'm sure the CPU defender has logic in place to not leave Klay or KD this open.