Within a monorepo like most code at Google, there isn't as much of a bright line between public and internal interfaces. If you don't get a public interface quite right on the first try, but it still has <~100 call sites, it's really easy to just make the change and fix all the callers. If your library has become popular and has substantially more call sites, then it's hard but still doable to fix.

This assumes that the interface is a function or method call within a binary. If it's an RPC, then making changes is much trickier, since you can't assume that both sides of the call were built at the same commit. This requires a lot of thought to make sure any changes to your RPC messages are both backwards- and forwards-compatible.