One thing I discovered: almost everything a manager does has a big lag between input and observed impact. Retention is one of the slowest, it's like steering a cruise ship!
So do not under-estimate the impact of retention.
I think GP was more saying that improving retention, at a % level, is a slow ship to steer. (At least what is available at the manager level.) Decreasing retention across the org in the long term doesn’t happen overnight, and it can take a while to observe whether the effect is trending downwards or upwards. (And also depends on what signal you use - people actually leaving, or people talking about leaving!)