It is 20.6, that's why it hasn't been 10 until it hits 19. There's a whole reddit thing too. This is all blown up nonsense and the operator screwing it up made it closer than it seemed. Then compound it with people on twitter jumping the gun (including their coach i think) rather than double checking.
https://en.wikipedia.org/wiki/Time_line_(basketball)
Google AI:
In NCAA college basketball, the 10-second backcourt violation is officially signaled when the shot clock displays 19 seconds. While some debate exists regarding clock accuracy, the standard, correct ruling is that 10 seconds have elapsed once the 30-second shot clock counts down to 19 (meaning the ball has been in the backcourt for over 10.0 seconds).
Key details:
The Trigger: The countdown starts when the team gains control in the backcourt.
The Visual Cue: When the display changes from 20 to 19, the 10 seconds have elapsed, and a violation occurs.
Context: This rule applies to both Men’s (30-second clock) and Women's (30-second clock) basketball.
Technical Note: Some, this Reddit user notes, argue that a whistle at 20 seconds is technically premature based on how 20.99 seconds is displayed, but 19 is the accepted, standard violation time.