Every metric is a proxy. Revenue is a proxy for value delivered. Churn is a proxy for dissatisfaction. Page load time is a proxy for user experience. The quality of a measurement system depends on how well its proxies track the underlying reality they are supposed to represent.
Technical decisions affect product metrics, but the relationship is indirect. A latency improvement changes page load time, which may change activation rate, which may eventually change retention. The chain is real, but it is also long, noisy, and full of confounds. Technical leaders who understand this chain make better decisions about where to invest.
The problem is not that metrics are proxies. It is that teams forget they are proxies and optimise the proxy directly instead of optimising the underlying thing the proxy represents.