Whether or not it's justified is a different matter, but for startups valuations are more about potential then current performance.
They made $25M from subscriptions one month, took that number, multiplied it by 12, arrived at $300M and everyone has been running with that line without ever asking what their churn looks like.
They could have churned $24M the next month, ask yourself why they are silent on churn if they are doing so well.
No, venture capitalists aren't ignorant, but their goal also might not be to build and run a healthy company long term. It might be to turn a quick profit by selling a startup to another company.