A broken culture would be expecting engineers to find every last bug in other people's code and blaming them for bugs.
Just trace the Whys:
1) Why did the bug happen? These lines of code 2) Why wasn't that caught by tests? The test was broken / didn't exist 3) How was the code submitted without a test? The reviewer didn't call it out.
Arguing about percentages doesn't matter. What matters is fixing the team process so this doesn't happen again. This means formally documented standards for test coverage and education and building team culture so people understand why it's important.
It's not "just the author's fault". That kind of blame game is toxic to teams.
In a good culture it's "our code" where everyone shares the responsibility for quality, performance, and functionality.