Fair point.

Though I feel like in any modern app deployment scenario that isn't going to be a meaningful defensible boundary.

The answer is to use neither, especially given the number of times I've used a path traversal vulnerability to expose /proc/self/environ on a pentest, and the one time I was frustrated on a pentest when every app in the environment used a dedicated API for secret retrieval and the path traversal vulnerability I had access to was worthless.